Mossberg 930 JM Pro Setup/Fixes

[IntenseImage]

Shot the 3-Gun match this weekend at Academi (Blackwater) and there were 4 JMs on our squad and 1 base 930 ... Only the base model had issues and guess what, he's never cleaned the damn thing! There was a Benelli shooter in our squad who had their performance 3gun model/package and it had an issue or 2.

Sent from my XT1080 using Tapatalk

[GunCat]

AF2

I've generally found no single issue on a problematic 930, its usually a combination of issues. The shell stop fix (from the Larry Week's Brownells article) is always a starting point and I see you have been in to that. Also the piston must slide in and out of the barrel ring without binding.

Check the area on the shell stop that interacts with the carrier on the trigger group. The edge of both need may need to be slightly deburred or a sharp edge broken to allow the lifter to move freely.

While looking at the shell carrier make sure the 2 legs that attach to the trigger group are straight and parallel with the body of the lifter ( I have seen some that went off to one side and held the working end of the carrier off center in the receiver.

With shells in the mag tube hand cycle the bolt (with the trigger group removed). Does the shell stop rotate and allow the shells to pop out of the mag tube every time? Too much mag tube spring pressure or a shell stop spring that is too weak (or a combination of both) can prohibit the shell stop from rotating. There is no “extra power” shell stop spring on the market (that I know of). I have had to make a few from spring stock to get reliable movement on the shell stop. This may require a bit of experimentation for custom spring length and pressure.

[AirForce2]

Thanks Steve,

I like the idea of racking a bunch of rounds thru without the trigger assy & lifter in the gun, hadn't thought of that for sure. Here are a few pics of the gun internals and even going excessive on the brand new shell stop assembly versus the brownells example in my opinion. The parts looked scratched up, but all of the internals showing silver are just the bluing polished off.

I also tried to setup the primary parts outside of the receiver but in fairly close to where they would be inside the receiver so people can kinda get an idea what's going on inside.

Wanted to add a pic of my tube to show some odd burn marks, but think I'm at my limit for pic sizes.

Img 102923-The bottom flat portion of the 2 piece bolt assy if you look close along the bottom side has 3 varying steps as the bolt comes back and seems to have 3 areas that change the position of the shell stop assy. The rear most portion of the bolt section 10% has a high side(when the bolt is forward/locked into chamber). This should keep the front shell stop into the middle of tube to keep shells from coming out and also keep the rear portion of the shell stop pushed outward toward the receiver wall. As the bolt it thrown rearward, the middle section of the lower bolt rail (70% or more) is mostly flat and a little lower and seems to cause a slight shift of the front part of the shell stop to partially come out from behind the live shell by the follower in the tube (does not full releases a shell yet) I think the front & rear shell stop assy are appx even tilt so to speak. Then as the bolt comes all the way back there is a slight dip inward (to the center of the chamber). This is what looks like the full release of the front shell stop that allows a round to kick backward into the lifter area and stops right in front or on the front trigger assy area and then the lifter flops upward after the small hammer sticking up in the rear of pic 102923. If you look at your part and pic, you will see I even shaved off a major portion on front of the trigger assy where the lifter swings up/down the night before the match just to see if this was interfering with the shell stop rear assy portion. Surprisingly it didn't make it feed any worse nor make the release of shells worse or better. I guess the rear of shell stop assy is supposed to sit against the trigger assy at times when the bolt is fully locked back or rearmost cycle of the bolt.

Edited May 21, 2014 by AirForce2

[AirForce2]

After reviewing my first couple of posts, it appears I was already having this (bolt cycles, ejects spent, gun goes click, shell stays in the tube) problem leftover after fixing the double feeds & bolt locking open w/shells in the tube. I just only had it a few times every 50-100 rds. Now after messing with it so much and can't seem to get any relief is why I'm hoping it's the gas system. Especially after putting in a brand new shell stop assy and this immediately gave me an additional malfunction back of "bolt locking to rear w/shells in tube".

I'm hard headed at times and to some, never should have touched it at all and let the factory do all the work. I like to be in the know and it makes me confident when I can narrow down a problem and fix/learn it in the process. If the engineers and Tech/Repair folks in the gun manf business put 1/3 of my efforts, thought, and care into a working and tested components, there would be a whole lot less bad guns and shooting related products out there to send back. My lesson learned: Be confident and have enough rounds thru the gun before adding your goodies and setup items to it. It was a long cold winter/spring so I did some work to make it better for 3 gun & my liking before shooting it much. Hope I get my stock & fore grip back, that stippling job and stock shortening was a lotta work.

I boxed up the JM Pro and shipped it back to Mossy with a 3 page typed letter 5/23/14.

I've also done a review online on:

-xdm .40 & xdm .40 comp (firing line forum)

-ruger SR40 & SR40C (If you own one you should read it) ruger forum.net

-Taurus 738 (.380) firing line forum

[AirForce2]

6/5/14-I received my mossberg back from the factory today. I may go shoot it tomorrow and waste another $40 between range fee and shells cost.

I don't feel it's needed to shoot the gun, here's why:

Upon inspection and reading the work order, the only think they did was replaced the "trigger assy". I sent them a long very detailed description of the previous and current issue with my shotgun and they replaced the part that only required replacing because I told them I modified the front fork in front of the hammer as a last ditch effort in case this was interfering with the rear of shell stop assy. I also told them this mod did not make the problem better or worse and was a last ditch effort to fix my issue is the only reason I needed a new trigger assy. I called mossberg and vented to the guy Ignaseo who works the phones in tech support and he took notes and said he would have the tech support lead call me tomorrow. We'll see if he calls me, if not I'll try to talk w/him after noon since I'm working a 12 hour mid shift tonight. I also sent them the gun cleaned fully & spotless. I won't do that this time if there is a 2nd return. I plan to insist on a return auth from mossbergs dime this time. I was pretty PO'd that either they didn't read the details or care enough to look at or replace any other components. I asked the tech phone rep if he could send me a new piston & the piston housing guts and he said he could only send me the piston. At this point I'd say send me a new barrel and piston & piston housing and a new shell tube. If that doesn't cure it, it's done.

Are there any mossberg reps on this site? If you are pls chime in. My only goal now is to get a working gun so I can feel good about selling it to someone else. This is poor service and design on a no so technical firearm and a company that has done nothing but shotguns primarily for years and still puts out such a hit/miss product. If our cars ran this poorly out of the box we tend to not be so tolerant, and cars are way lots more technical. For some reason bad guns are accepted and protected by many as "it happens" or "it's the shooters ammo/fault".

Here is the major portion of the letter I sent them with the 1st return:

-------------------------------------------------------------------------

Mossberg Arms Service Dept,

PLEASE, IF POSSIBLE RETURN TO ME BELOW ITEMS, THEY COST A LOT OF MONEY & TIME TO MODIFY FOR MY FITMENT:

- MY ORIG REAR STOCK, LIMBSAVER RECOIL PAD, AND STOCK SCREWS, INNER NUT & ALUM SPACER.

-FRONT HANDGUARD

-LIFTER GATE

Ser#:

I use Briley Ext chokes and can’t find my orig flush chokes, so insert one before testing pls.

I’m returning this problematic 930 JM Pro, 22 inch (700 rds thru gun) in hopes that you can make it a reliable weapon. I bought this brand new in Late Dec 13, but did not get around to shooting it until mid Feb 2014 for the first time. Since this time I’ve had a multitude of malfunctions in the first 200 rds and fixed most except one particular issue has grown noticeably worse after 400 rds. I was able to reduce the tube spring length, and reshape the shell stop assembly to reduce a majority of the issues that seem to plague this platform but one still remains:

When gun was new:

*double feeds

*bolt locks open w/live shells still in the tube

*shells don’t feed from the tube yet the empty shell extracts but bolt closes on empty chamber

*major scarring/gouging a line into the shell holder tube from the top thread that connects the ext tube down thru the piston assy after only 100 rds. I had to use 2,000 grit sandpaper to remove most of the gouge and also slight debur an area inside the piston housing (an o-ring) of the barrel to keep it from ruining the tube.

*Shell holder tube & ext Nordic tube joint misaligned-causing follower to kink at the junction.

NOW:I was able to get rid of every malfunction except one and this has grown from a 2-5% per hundred rds to occurring appx 15% to 30% of the time, regardless of soft loads, medium, or heavy loads.

1. Live shell will not release reliably from the tube during firing and the empty casing ejects but will not feed a live shell out of the tube and bolt closes on an empty chamber. Each time this occurs, a hand (manual) cycle of the bolt rearward releases the next live shell from the tube and feeds correctly. Even if all gun parts disassembled & cleaned/lubed, Gets much worse after 20-50 rds. After 100 rds was happening 20-30% of the time but only 0-1 in first 20 rds after detailed cleaning of entire gun. Gas system?
2. I need a new trigger assembly-My last ditch effort before a major match and last time I shot this gun, I shaved off the front portion of the area at the back of the lifter (front most portion of the trigger assembly) to see if this was interfering with the rear shell stop assy. This did not make the malfunction any better or worse surprisingly

-It does not seem to be better nor worse with a full, half full, or a few rounds in the tube.

-It is not dependant on the power of the load used.Most have been 2 3/4 , 7.5 shot, 1 1/8 ozand some 1 oz shot loads varying in speed from 1150 – 1300 fps.

-It appears to happen only 1 or 2 times first 30 rds after a full gun cleaning including a detailed carbon removal from the piston housing & piston & rings (removed for cleaning) and inspection after cleaning.

-There are weird marks on the tube portion that sits near the gas holes of the barrel (blackish, burn scores).Since this malfunction seems to grow at a higher rate every 20 rds or so after detailed cleaning, I suspect something in the gas system/area. Sometimes it will fire 8-10 rds fine but most times will jam each round or skipping a few rounds.

-Does not matter if pistion and piston housing are dry, dry lube/light lube only on piston & piston housing, wet or very wet lubed piston/piston housing.Also doesn’t matter is rest of gun is dry, mediun, very wet lubed.

I was preparing for a major USPSA shotgun match and knew I couldn’t send the gun back and returned before so after hours of inspection of all parts inside, and deburring any/all sharp edges had to attend the match with terrible results.

I spent a lot of money in May due to this platform:

-$100 on 300 plus rds of ammo function firing/range fees 1-16 May

-$45 replacing parts I ordered from Mossberg a few weeks ago, no help. These items were:

- the recoil spring in the rear stock

- shell stop assembly & shell assy spring: Non of these helped reduce the primary malfunction but a new shell stop assembly did give me a return of an additional problem that I’d gotten rid of previously so now the gun bolt was locking back with rounds in the tube and also still cycling the bolt closed on empty chamber, but round/rounds still in the tube. I had to reshape/debur a brand new shell stop assy just to get back to one type of malfunction.

- forward shell tube spring.

-$200 cost of major shotgun match entry, ammo, gas, prep. Gun functioned terrible and had to hand cycle 30%-50% of the rounds by end of match and had to DNF last to stages because tired of hand racking and when gun goes click cost me tons of failure to engage & missed activated targets including risk of getting unsafe towards a DQ. If I can't trust a gun in competition, I sure can't trust it as a defense weapon either.

----------------------------------------------------------------------------------

I have a solution in the works thanks to Mossberg's assistance. Just sucks it's cost so much in comparison.

Buds Guns shop: $1350 VersaMax Comp Tactical is enroute

[AirForce2]

I called Mossberg and talked with claimed service tech lead (Joe) and discussed my gun issues and the hit/miss functioning 930 JM Pro guns out there. I also explained the multiple issues and malfunctions that are out there as not acceptable for a defensive or sporting weapon. I basically said I feel the JM Pro is a coin flip and that if they don't address the issues to make this gun more reliable, the sales will be good for 2 years and then these sales will go down a lot more once there are to many malfunctions and problem guns out of the box for people to deal with.

I asked him after describing the fixes I've done and the lingering malfunction of cycling spent casing and then closing on empty chamber rounds stay in the tube and hand cycle works every time, what made them center on replacing the trigger assy, as a cure for this malfunction. He claimed there were some trig assy's that had issues on some models yet didn't really seem to know what the difference was. He seemed to care and said he had seen my gun and all the extra 3-gun mod work put in so decided to leave the carrier on the new trig assy.

I looked today to a physical difference on the trigger assy they gave me and guess what at least one difference is mod is?

The same think I tried to do night before the pike peak challenge listed in my earlier post. The new mossber replace trig assy has a decent portion cut back on the front forks of the trigger assy portion. This is the channeled area right behind where the lifter swings up and down to clear the front portion of the trigger assy. Also same area in front of the hammer if hammer was vertical as viewed when trig assy is out of the gun. They also put a notch/groove vertically and cut into the shell stop assy side of the trigger section. This is also where the shell as it comes rearward from the loading tube would stop before the lifter flops it upward to the chamber. This is the same exact thing I did on my gun as a last ditch effort and it did not help at all. The new trig assy still shows the rear of the shell stop assy hits right on the new notched area and actually rides across the front ledge ie..shell stop rear portion still scapes the front of the trigg assy sectionfrom the little bit of cycling I've done. This new trigger assy portion was "factory", it didn't seem as if someone had done this on a factory part and was blued. The new trigger assy does seem a little tighter side to side and movement in the receiver area.

I'll let you know how the testing goes.

[AirForce2]

I think Mossberg thinks this will make the shell stop swing in/out front & back portion more freely, or prevent binding in the rear shell stop portion. The rear portion must swing inward toward center of gun in order for the front shell stop portion to swing outward and release a shell. Problem is the front portion of shell stop only swings outward to release a shell as much as the bottom most cut on the bolt rail angle will allow because the bottom rail cut angle ride againse the flat ledge sticking out the top of the shell stop assy. The new trigger assy is a little tighter in the receiver than my other one was so I'm not sure if there are any other dimensions different on the new one. My trigger feels a little stiff, but likely a few shots will be good again. I forgot to take out my ruger 10/22 spring from my old trigger assy, so I lost that but still have one extra. I'm gonna run the stock one and if I get a light primer strike, then replace it.

Actually a shell kicking out of the tube to the rear part over the lifter and stopping against the rear portion of the trigger assy also must keep the rear portion of the shell stop assy from locking the gun open. I don't have pics, but the old trigg assy I removed 1/8 to 1/4 inch across the front trigger assy and also still had to shave down the side portion like Mossberg did on the new one. Be careful not to just cut the front face only on one side (I don't know if section is likely where the shell stops against just before lifter throws it up to chamber or if the rear vertical notch in shell assy does some of that)

You guys with this type of malfunction, look at my pics and compare the new mossy trigger assy front area with yours and see if your's has a lot more meat across the front trig assy face just past the circle where the hamper spring would get inserted under the hammer or if has a notch cut into yours like the above pic. Mine still looks like its cutting a silver nick right at the front corner from a bunch of hand cycles. I'm going to put 50-100 rds thru it tomorrow and see.

Edited June 8, 2014 by AirForce2

[Who Knew]

Looks like one of the mods I've been working on. I filed a little off of the trigger assembly to see if it would allow the rear portion of the shell release to pivot inward a little more. It seems to be working but as you can see from the pic it's still hitting the trigger assembly. I think I'm going to take a little more off.

[AirForce2]

Shot 50 rounds out of it today and it ran them all fine. Herters 7.5 shot, 1 1/8 oz, 1200 fps. Better than it was before. Something in the new trigger assembly must have made it better. I haven't gone 50 rds without a jam in the prev 400 rds. Nothing changed or added since I got it from factory. They didn't replace my tube spring, recoil spring or anything else.

Edited June 9, 2014 by AirForce2

[AirForce2]

70 more rounds today of estate comp 7.5 shot, 1 1/8 oz, functioned flawless. I don't know if it's just the notch in the new trigger assy fixed it, but it's finally shooting good again. I had my old trigger assy with a similar mod and I'm confident the shell stop assy wasn't touching the trigger housing, but maybe I was close . I tried to look at my other pics of the trigger assy but don't have any pics from the front or top view to be able to tell if the new one vs old one has a shorter front trig assy fork. I still have the #2 shell stop assy that I tweeked in the gun and I did measure both and there was not dimensional diff on shell stop assy's. Thanks to all for the assistance. Hopefully this thread will solve some issues.

Edited June 9, 2014 by AirForce2

[AirForce2]

Kinda funny, past 2 days had good ending for my JM Pro and tonight I got confirmation my Remington Competition Tact is being shipped to my FFL. Just gonna shoot the Rem a hundred or so rounds before I do to much dicking with it ie...gonna have to cut down the stock shorter, add an adj rear sight, maybe stipple, likely have to fix the shell stop (hear shells are a PIA to load already), weld up or shave down lifter (heard loading double/quad loads due to lifter is iffy). Good god, can't they listen to people and test & develop before they produce anymore? Maybe I'll do a post on the "competition model" later.

[AirForce2]

Well, maybe I'll not ditch the mossberg just yet. Haven't shot the Rem Versa Max Competition Tactical yet, but let me just say how sh_ _ _ y it is to load a shell into the tube. Amazing that they let this gun leave the factory and I know it was fired, which means they had to load the tube. The shell is so hard to force into the tube once the brass casing hits the hump on the outside of lifter/carrier and then excessive pressure from the lip of the shell catch once you get it past the lifter, that the sh_ t_ y designed short lifter will snacth the crap out of your thumb. I only loaded 2 shells.

Took it apart today and did a quick grind down on the rear and sides of the foregrip for load 2/4's and this should be ok. Took down the ears right at the 10, 2, 4, and 8 o'clock positions right at the junction of the shell tube and load port. Polished & rounded the chamber entry lip & extractor area on the barrel. Also made enough room on polishing down the ext/outside hump of the lifter and a hair off the inner front lifter legs that contact the bolt bottom and a slight rounding of the front shell latch corner on the inside edge and corners and now the lifter doesn't cause any drag during loading a shell, but loading a shell into the tube is still excessively hard and then the crap lifter grabs your thumb due to the shell catch lip catching on the brass of the shell. Unfriggin believable. It's a pain to load a full tube for sure, and the Remington folks that test it must know it, because I've seen reviews since last summer that the competition model gun magazine etc.. reviewers told them it was to short and they were supposed to make the lifter longer.

The lifter on this shotgun sucks worse than any gun out there because it's too short. At least other guns lifter ears would be long enough to go close to the tube opening so that only the middle of the lifter would catch your thumb. This thing is straight across but a good 1/2 to 3/4 inch to short. The trigger feels like a mushy old rusty grenade pin being pulled. I'll have to say the mossberg trigger is sweat in comparison.

Goods:

-Nice steel shell tube and the carbon sleeve is also the spacer for the end cap to hold the foregrip tight.

-+2 tube ext is not steel inside, so hope the carbon is good enough. It's mostly only going to house the spring and part of the follower with 10 rds loaded, so little risk to hamper feeding. Carbon Arms is the tube I think and it's nice & light.

-Fit a finish of most of the gun seem good and came with 4 ext tube chokes which is nice assuming they work good and don't need to buy briley's.

-Internal parts and ext had no major jagged edges that I could see.

Bads/not impressed:

-The lifter sucks (see my comments above in this post). PO'd me off that still need to spend $50 or more to fix their crap design. It's worse than typical remington model shotgun lifters. Remington, pls tell the under 10 year olds/xbox shotgun design team that must have the old blind persons, people who've never shot or used shotguns before to please run their finished design/product thru the "NON blind" QA dept, and maybe someone who knows how a firearm should function before blessing the production. I know they had the right people to give them inputs.

-Throwup green receiver, whatever. Might have well made it glow in the dark zombie green.

-Trigger pull-feels like it's designed for a jittery 8 year old to not go off if they were having spaz attacks or stroke. Nice smooth 8-10 pds feels like.

-Shell catch-loading shells is like stuffin a round peg thru a sqround hold. You have to push so hard to get it by the shell catch and then the short lifter grabs the heck of the thumb becaue you had to force so hard your thumb goes way to far into the tube. Who friggin designed this thing "an old world beaver trap maker".

-The rubber tape or whatever is on the stock handle and foregrip-not so grippy and likely just gets in the way or slippery when sweaty or oil. Should have just made the finish rough or left alone so a $3 soldering iron & tip stippling can be done. Not sure if I can stipple the rubber areas or if the rubber can come off.

[AirForce2]

A word of caution since my orig post on doing the load port expansion. In the pics of my orig load port opening I angled the front portion of the load port away from the shooter towards the front of the gun. This is incorrect. You want to open up ie...shift the front load port opening forward but don't angle the opening away (angle the opening back toward the shooter) Open the port almost to the section where the shell tube meets the receiver around 1/8 inch or a little more so your thumb doesn't have to go up/into the shell tube to much. Just make sure you angle the front load port back towards the the rear of the gun/toward the shooter. If you don't you will likely find during load 2 & load 4 that the second shell tends to ride up and jam against the front load port cut. I salvaged it a little and now know I need to correct this better on the mossberg. Hey, gimme a break, it was my first shotgun. Other than the lifter weld up that I can't do, from pics I've seen my work looks top notch for a rookie. If I'm unable to post the pic of my mossberg and the better port job I did on my versa comp here, just look at the diff on my versa competition post and you'll see what I'm talking about.

PS I also suggest for load 2/4 you may like removing some of the material in front of the trigger as in the versa pic. This allows the 2 shells to lie straighter as you start your load. I had this mod on my orig trig assy but since mossberg replaced my trigger assy, I haven't done it to this one yet.

Edited June 29, 2014 by AirForce2

[AirForce2]

The 930 JM Pro that started this topic is now for sale. The post can be viewed in Denver Armslist. It's in the same "working", "reliable functioning" state as it was on my last few posts after the trigger housing was replaced about half way up page 3. I function fired 50 rds again last week and still works and has had only around 150 rds since it was fixed. If you are in CO and trying to get a cheap working 3 gun shotgun, it's avail. I was keeping it as a backup and mainly just cause I put so much love & hate into it and learned a lot and finally it works good once the one feed from tube issue was resolved by a redesigned trigger assy replacement, but could use other toys and maybe someone else can make better use. Price is $750 firm.

Edited February 20, 2015 by AirForce2

[AirForce2]

Sold. Kinda feel like I lost a real hot girl. Spent a lota money on her, made er pretty, fixed some cosmetic flaws and high grade parts, finally had to get physical, then couldn't trust her and had to let r go. Halfway to a country song

[swhiteh3]

Ext Tube: Many complaints of the tube connect joint is uneven. This may/may not be a one item fix. Mine didn't appear to jam during live fire, but it's like having a bump in your piston wall (can't be good, can only hurt). With barrel clamp, tube ext all tight, remove the end cap (buy a Nordic XL for +1) and w/flashlight look down tube and rotate looking for high spot typically on on side only. Determine if it's the chromed tube of the Mossberg sticking up or the Nordic ext tube and you can dremel/polish this side down and try to loosely slide spring/follower down. Some have said to loosen the Nordic tube nut and rotate the ext tube and this may even up everything, I don't like leaving it loose, but also ensure the barrel clamp is not causing the issue by pulling/pushing the tube up or down causing the alignment issue. In short a combo of sanding the angle of the front foregrip/stock where the nut tightens, polishing down the rear chrome tube and minor polishing of the underside of the Nordic barrel clamp is what mine took. What a pain in the rear. I also have a GG&G follower, but others should suffice.

Well, I've never had an issue with this before om live-fire, but tonight while servicing my shotgun, I realized that my follower does not drop freely down all the way. It stops at halfway (the joint between the magazine and the extension) and usually "bounces" the rest of the way down. Obviously, not ideal. I can't tell where the bind is coming from. I've slightly polished the leading edge "corner" of the follower, and cleaned up the leading edge of the magazine tube, with no improvement. I've also played with different "torques" on the locking nut. Looking down the tube, I can't see anything obvious. Any ideas? I have a match on Saturday... and I'm a little concerned how it's going to run.

-Scott

Edited July 17, 2015 by swhiteh3

[mpom]

So did it run?

Your situation is not unusual. Typically spring pressure and recoil overcome the step and shells feed right. Placing a slight radius on leading edge of follower makes good sense.

I've opened up the mag tube mouth slightly to create a bit of a funnel, no issues so far, but running a shortened Nordic follower.

[swhiteh3]

Thanks for asking. Yes, the gun shot flawlessly. The driver? Not so much! :-)

Seriously, it fed just fine. I put a chamfer on the follower, but it's so thin you can't get too aggressive with it. I'm planning on buying an aftermarket follower that I can put a generous radius on.

And of course, that helps the follower move freely but does not keep a shell from hanging up on the lip, so after the match I took it apart again and chamfered the magazine tube a bit more and polished it. It's far from perfect. It's amazing frankly how bad the inside diameter mis-match is between the magazine tube and the extension tube.

[swhiteh3]

I had issues when I first got my 930 JM Pro. Failure to feed, failure to eject, and bolt locking back. Talked to some shotgun gurus on another shotgun specific forum and was recommended that I "fit" my shotgun. After some back and forth with them asking me questions, they told me to install the drop spacer that came with my shotgun. Issues went away immediately, and have never come back. I have let a couple of friends use mine since installing the drop and they have the issues when firing it since it's fitted to me.

Check to see of your shotgun is fitted right, when quickly going from low ready to a firing position your head should naturally be lined up for you to look down the sights. If you have to adjust to head position when doing so, it's not "fitted" to you, and you need to experiment with the shims that came with the gun.

The more I shoot mine, the more I realize that my gun does not fit me well. When I first shoulder it, I see too much of the top of the rail, so putting the dot on a target and pulling the trigger will always send the shot high. I've dropped the comb as much as the stock spacers allow, and it helped A LOT, but it's not enough - I need more. Any ideas?

[BitchinCamaro]

How much further down do you need to go?

You can take a heat gun and soften up the top of the comb. It's sort of "peakish" anyway, so flattening it out will give you 1/4" of drop without too much issue. Much more than that the gun is gonna get ugly, but at least it will fit!

[JayDee]

The more I shoot mine, the more I realize that my gun does not fit me well. When I first shoulder it, I see too much of the top of the rail, so putting the dot on a target and pulling the trigger will always send the shot high. I've dropped the comb as much as the stock spacers allow, and it helped A LOT, but it's not enough - I need more. Any ideas?

I had the same issue with my JM Pro. Easy fix if you just need a little bit more than the drop shims can do: take off the recoil pad and add a couple washers under the top bolt that attaches recoil pad to stock. This was just enough to bring my the rib right to my eye.

[JayDee]

I also just got my JM Pro back from Mossberg. After about 250 rounds it started with all the issues many have described: bolt locks open with mags in tube, shells getting stuck on lifter. I replaced my mag spring, added a low drag follower, polished/deburred the edge of the mag tube, and then aligned the tube extension and tightened clamp w/o the extension being cranked down and out of concentricity. For a while it helped, but then in my last match I had 5 jams on the shotgun stage (40 rounds).

I contacted Mossberg and got return instructions. I had to ask for a FedEx return label, which they provided after some polite demanding.

Like Airforce2, they replaced the trigger assembly & also my lifter. There was no other communication about the issues or why.

I just went to the range on sunday and ran 50 shells (Fed target) through it rapid fire to see what would happen. Happy to report that it ran 100%. Will have to run a few more boxes before I take it to a match. It will be good to have this thing running again, even as a backup. My new Versamax is almost ready… been at Carbon Arms for the last couple weeks getting tricked out. Win win!

[savagemann]

I recently picked up a 24" JM 930 pro.

I did a lot of research before buying.

I ordered the or3gun MST, recoil spring plunger and follower before I even had the gun in hand.

I never even shot it stock.

When it showed up, I completely stripped and cleaned it. Installed the new parts and it has been running good.

Although, yesterday I was messing around shooting trap and had 1 light strike.

I DO have the 10/22 spring on hand to fix that potential issue, but haven't installed it yet.

[twohawks]

Just came in from shooting my new out of the box (except I completely tore it down and cleaned and lubed according to my preference) Mossberg 930 JM Pro. Fired 5 rounds of Remington slugs through it to sight in the cheap Konus Red Dot sight. No issues, dialed it right in. Then a 5 box of Remington 00 Buck. Still no issues. Then a couple of boxes of cheap #8 Fiocchi ammo dove loads. Shot some rapid fire strings of 9. Flawless.Then I got out a box of old ammo that was given to me by a guy that had had the rounds since the 60's. Mixed rounds...including some paper shell 7 1/2's. Ran 22 rounds through it as fast as I could pull the trigger, I'm no slouch FWIW, and the gun was always waiting for me to fire the next round, no malfs at all. 

I am posting today to confirm that while apparently there are some lemons that come off the Mossberg assembly line (like any other company), maybe a few more at Mossberg than from the Benelli factory,  I am glad I went the JM Pro route (and load up on farkles and ammo with the $$$ savings). I was indeed apprehensive to get one of these based on various reports. This gun handles very well....recoil certainly lighter than my pump guns, and it looks really badass. I am not certain if it will handle everything manufactured, like the 900 fps lady defensive stuff. But I have some OR3Gun parts ordered that should take care of anything else. 

After posting this originally I decided I wanted to shoot some more so opened a box of Federal Multi Purpose Walmart bulk and a box of Winchester Universal Walmart bulk. So much fun! As fast as I could shoot...Same results...no issues at all. 

 

TH

Edited November 11, 2016 by twohawks

[tact]

I have had no problems with mine either. I'm more than happy with it.