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Mossberg JM Pro Failure To Feed


ladysaiga

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I had earlier issues with my JM Pro when I very first got it and figured out what many have stated here. Cleaning was the key to reliability. I ended up getting into a routine of scrubbing the outside of the mag tube with 00 steel wool and a non petroleum cleaning solution with a little water, wiping it dry (lube only with a little dry silicone wipe). I took a page from Jerry's youtube video in cleaning the 930 and got a set of these 18 inch handle bronze bristle bottle brushes. They make cleaning the inside of the spacer, piston bores so much more efficient and easier. I run Winchester super target loads no issues.

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As for opening up the loading port here is what I did myself. I load deuces primarily but quads are not a issue either.

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That's some impressive loading port work!

I'm curious as to what your 930's forend modifications look like for the quad load technique. I'm right handed, but VERY much left eye dominant so I have always run long guns left-handed. I'm pretty proficient with 'weak hand' (strong hand for me, really) and have developed loading port mods that work great for me. That said, I'm now looking to set a 930 up for my wife. She will probably be ahead to run load two or quads, so I may modify one in favor of those techniques. I like the wide port cut and welded lifter like you've done, but it seems like a stock forend will require major surgery for a smooth quad technique.

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Any more reviews on the marine spacer, I'm big on cleaning mine the way jerry miculek recommends but I like the idea of the new spacer. Ideally I would like to see someone do the same concept just Teflon coated.

Put one in both my 930's.

Works fine in the JM Pro w/ everything (see: above).

In the XPS it wouldn't cycle the low-power (980fps). If you look at both gas systems you'll see the XPS is a bit different. It works fine w/ standard 1200fps target loads . . . so actually no problems.

If you install one you'll see it's a loose fit, so my guess is Teflon not needed.

I do have one concern: it is aluminum & can it take the pounding of thousands of rounds? Guess we'll see "on down the road"!!!

ps "One Shot" dry lube

Edited by stinsonbeach
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Interesting, I have both as well. The spx mag tube is not polished, is that the difference you mention or something else? Did both have issues before the spacer. ?

SPX has a different jet up x the piston. Leaves a bit of a "scorch" mark not evident on the JM. I did't look that closely so it might just be my quick look. Anywayzzz the XPS wouldn't cycle the 980fps'.

I had NO problems w/ either of them before - after I ironed out a few spring/feed problems - but thought the Marine Spacer made sense as I said, it was to be a "go-to" besides 3-gun.

I'm convinced it is money well-spent - IF it holds up! IF it were just a SHTF gun I wouldn't think twice, but 3-gun pounds the crap outta' that spacer. Good testing ground!!!

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I can't see it being any less durable than the factory part

Ditto, as I believe the factory part is also aluminum. That said, not all aluminum alloys are the same, so hope the Marine Spacer is heat treated and hard anodized, as I bought one, but have not yet received the gift to myself :)

Edited by mpom
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Lots of topics in this one topic, but I guess it all falls under the heading of JM Pro failure to feed. A lot of this has probably been discussed before, but it seems like a good place to cover some of it for everyone else. At some point, maybe we need a thread on the part itself to keep from derailing this one.

The topic of WHAT we're all asking the 930 to feed could be its own separate tuning thread. The Marine Spacer Tube is designed to address a specific problem that occurs in many environments. Moisture. It does a great job at that and allows guns that would run in the Arizona sunshine in summer to run in the middle of Oregon's winter. It does this by nullifying the effect of hydraulic friction and the build up of sludge caused by cycling the gun in high humidity. This is what allows the 1200 FPS bulk pack staple of 3-Gun to work in these 930s that are finicky to the buildup of moisture and sludge. There are other benefits to the part, but we wouldn't have developed it were it not for the moisture issue itself.

To understand what the Marine Spacer Tube WON'T do, we have to dissect the rest of the gas system and components. An SPX has only 8 inches of barrel past the gas ports. The JM Pro has 14 inches past the gas ports. The JM Pro has a more forceful gas impulse of longer duration as a result. They naturally run better at the lower limit of available shot shells. A properly maintained 24" barreled gun will reliably cycle shells that an 18" barreled SPX won't. Finding the 'floor' of each gun will depend on the tolerances of YOUR specific gun. The Marine Spacer Tube will help your gun reliably operate shells that won't run reliably without it. The 'random' malfunctions caused by sludge and moisture buildup simply don't happen anymore with the MST.

The natural limit of a perfectly DRY Mossberg 930 is going to vary based on unrelated components. For instance, the force behind the gas piston is responsible for pushing +/- 5 oz of parts against a +/- 7 lb spring before the pusher assembly even reaches the bolt carrier (or slide assembly). When it reaches this point is has to do it with enough force to slap another +/- 11 oz worth of bolt, carrier and bolt handle against a +/- 15 lb recoil spring hard enough to travel all the way back to the ejector. This doesn't account for whatever force is required to extract the spent shell. Shells that have weaker hulls (Winchester Universal) are deformed more than stiffer hulls (Winchester AA) when fired and are tougher to extract, absorbing more of the gas system's energy. That's a lot of +/- going on.

The 930 was designed with a secondary gas system that lives in front of the serviceable piston inside the barrel assembly. When the gas pressure builds beyond a certain point it will push this internal, captive piston (with single ring) the opposite direction of the gas piston assembly until it reaches the large vents and expells the 'excess' gas.

The limits of what a given gas pulse can do are ultimately affected by the pusher spring rate, recoil spring rate and component weight. Unless you have a burr, binding or other issue, these items are constants within your gun, but not necessarily from gun to gun. 980 FPS shells simply can't overcome these factory parts in most 930s. The key benefit to the Marine Spacer Tube is that it greatly reduces the resistance to movement of the gas system. If the primary gas system is too resistant to movement (due to hydraulic resistance and sludge build up), the back-pressure opens the secondary gas system and vents the gas that is needed elsewhere to drive the primary gas piston with enough force to do its job. The MST regulates this concern in an otherwise smoothly functioning gun.

The Marine Spacer Tube is the first of three parts we're working to refine for the Mossberg 930 at this point. The second two will compliment the first. The manufacturing arm is also continuing to refine the MST as we pass on your valuable feedback. The machinists that manufacture the MST for us aren't new to gun parts. They produce a number of them for a variety of companies. The alloy chosen was tested successfully with far thinner tubing walls. We found the point where it failed and worked from there. Suffice it to say, attempting to create one of these on your own from the factory part is an extremely short-term solution to the moisture problem. Some day I will dig up a picture of how that will work out for you.

We have run the opposite (to 3-gun) extreme of shells with the production MST to discover that a properly functioning 930's secondary gas system starts to implode before our finished part shows any weakness. A case of 1600 FPS slugs as fast as you can load them isn't likely to be encountered in a 3-Gun match, but an MST straight out of the production run did that too without a hiccup, AFTER some 400 rounds of bird shot and other random stuff. Unfortunately, the single gas ring in the 930's secondary gas system (further down the barrel) didn't approve of the diet and gouged the snot out of the magazine tube. This gun will now be getting a new, Boron Nitride magazine tube from a JM Pro because it no longer seals tightly against the secondary gas system. Because of this severe scoring, and resulting leakage, the primary gas system doesn't have as much gas to work with now under 1200 FPS conditions. This is likely an example of running outside the expected parameters of Mossberg's design. The gun appears to be better suited for 3-Gun and waterfowl hunting than a slugfest, minus that pesky factory spacer tube that fills with sludge.

If the desire to run 980 FPS shells is a primary concern, adjustments will have to be made to other parts in the gun. Aside from an 'all clays' stage, 980 FPS doesn't appear to fit most 3-Gun needs. For starters, steel doesn't seem convinced it has been shot with it. We ran another 150+ rounds through the abused gun yesterday to see what it took to make a brand new MGM Spinner go. At 15 yards, it took 6 rounds of 1200 FPS through a Cyl bore. It took only 2 rounds of 1300 FPS shells with an IC to make it go. 980 FPS could be reliably cycled with a lighter recoil spring an a couple of other tweaks, but then DIFFERENT parts may get beat to pieces if you decide to run a case of 1600 FPS slugs.

There are always weak points in any system that can be found when something else isn't right. Early 930 SPX models were notorious for having out of spec springs in the secondary gas system. Too strong of spring and the rest of the gas system gets beat up as a result. Too weak of spring and the secondary opens up to vent the excess that really isn't excess. The MST helps maintain this delicate balance and turns a 930 that always runs right in 40% humidity to one that always runs right in 100% humidity. After our spinner session yesterday, the inside of three 930 forends were filled with sloppy, wet carbon. It was actually sunny out, but there it was. The MSTs wiped clean easily and the magazine tubes under them were astonishingly dry and clean. One of these times I will remember to take some pictures of a 400 round session to show everyone the difference. The anodized finish is as close to teflon as it needs to be. It cleans up easier than the Boron Nitrided parts themselves. The MST also has 75% less surface contact with the magazine tube, so there is little benefit to it being any slicker.

As far as the MST standing the test of time, we are 100% behind those already out there and refining as we go. We take it upon ourselves to try and stay ahead of any feasible round count they may experience outside of our hands. We pulled a bunch of our initial production run and turned them into ornaments because of minor differences in machining and finished weight. We're talking about less than 3 grams worth of variance, but know that translates to a thinner wall somewhere. Our alloy and tempering choices may also evolve if we determine they will make the part even better suited for the task. The target for the MST's durability is such that if one of them fails, it will be because something else went wrong in your gun and the MST absorbed the hit for you. That's when you can send it in and we'll send another one right out for you to install after you figure out what ate it.

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I can't see it being any less durable than the factory part

Ditto, as I believe the factory part is also aluminum. That said, not all aluminum alloys are the same, so hope the Marine Spacer is heat treated and hard anodized, as I bought one, but have not yet received the gift to myself :)

When you do compare it to the stock part...

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The Marine Spacer Tube is the first of three parts we're working to refine for the Mossberg 930 at this point. The second two will compliment the first.

You have my full (and willing to test) interest [emoji2]

+2 . . . & you ("logiztix") seem to have answered all of my concerns.

As stated, the SPX will not cycle the 980fps load, but the 1200 runs fine.

The JM Pro however, does cycle the 980's. Knew there was a difference in gas systems, but didn't look close enough - you've explained it.

As I'm going to grab my JM if I have to head out the door, I'm going to be watching this thread closely for updates, malfunctions & part failures. Running 1600 1 1/8oz slugs - or higher - & 1600 4-buck, is an imperative for a "go to" gun.

As of now, I have one in my JM & another in the SPX & looking forward to the other "two" upgrades.

I'm going to run them hard & if there is any problem, I shall let you know "soonest"...

Edited by stinsonbeach
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Another thing to check on your 930s when you clean them next can be the culprit in diagnosing why lower FPS shells won't reliably cycle AT ALL in your gun. I've seen this on a couple of earlier 'Eagle Pass' guns (both SPX models), but have yet to see it on any 'New Haven' 930s.

With a clean, disassembled gun, check that the piston slides smoothly in the cylinder by itself. Next, with the piston removed from the cylinder, check that it slides smoothly over the entire length of the magazine tube. Then, with the gas system and barrel assembled to the gun, park the buttstock on a counter and pull down on the piston through the end of its travel. This should also go smoothly. If it doesn't, or gets tight about half way through, you have binding. On a clean gun, the force of the pusher assembly spring will usually return the piston into the cylinder when you let go. If this doesn't run smoothly for you, your cylinder may not have been welded in perfect alignment with the barrel. This will drive all of the gas system components at a bit of an angle, which will really slow things down and beats on the internals at the same time.

This is critical enough to reliable function that I would remove the forend from any 930 and check this before I bought it.

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I'm a new JM Pro 24 owner and just got out to shoot mine yesterday after doing a through strip/cleaning. Conditions were wet with considerable drizzle and fog. Only put about 100 rounds of cheap 1290fps Herters #8 field loads from Cabela's through her shooting clays but function was 100% with no issues whatsoever. I had installed the Marine Spacer Tube from Or3Gun so maybe that helped?

My brother was also shooting his more seasoned JM Pro 24 that has probably 750-1000 round count through it and had no issues either. FWIW, his gun also had the MST installed.

Sent via my Nokia Lumia 520 with Tapatalk

Edited by NorCal707
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  • 2 weeks later...

Now I have about 5 things to play with when I take my new 930 out... Depending on how I feel when I get up in the morning, it will be exciting or tedious. :D

Well, which was it? Exciting or tedious?

I spent part of the holidays with files in hand, modifying two 930 loading ports for twins/quad loading that were previously opened for weak hand loading. I'll post up some pictures of the finished projects later this week. I used a blend of good ideas from this forum, a variety of stunt thumbs and some new Taccom shell holders for final tweaks. I must have taken the first 930 apart 15 times in the process. That was tedious, but coming up with a port and forend profile that works for both loading styles is exciting, I guess. :mellow:

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Now I have about 5 things to play with when I take my new 930 out... Depending on how I feel when I get up in the morning, it will be exciting or tedious. :D

Well, which was it? Exciting or tedious?

I spent part of the holidays with files in hand, modifying two 930 loading ports for twins/quad loading that were previously opened for weak hand loading. I'll post up some pictures of the finished projects later this week. I used a blend of good ideas from this forum, a variety of stunt thumbs and some new Taccom shell holders for final tweaks. I must have taken the first 930 apart 15 times in the process. That was tedious, but coming up with a port and forend profile that works for both loading styles is exciting, I guess. :mellow:

Too cold to be outdoors :| Unfortunately this week we've had both cold and snow. If it were just cold I could just light a fire, and if it were just snowy I'd bring a rake for my hulls, but both at once? No thanks.

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"logiztix" you were talking earlier of 2 more tweaks for the 930 beside the MST.

Hints?

Progress?

One of the two new parts is going to the anodizer this week and will hopefully be ready by the week of the SHOT show.

The other shows up as a prototype in the pictures I took of the loading port work I just finished on a couple of 930s. I haven't posted the pictures of the ports anywhere yet because I don't have a solid quote from our machinist to determine the feasibility of offering the clearly visible part.

Does that help?

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"logiztix" you were talking earlier of 2 more tweaks for the 930 beside the MST.

Hints?

Progress?

One of the two new parts is going to the anodizer this week and will hopefully be ready by the week of the SHOT show.

The other shows up as a prototype in the pictures I took of the loading port work I just finished on a couple of 930s. I haven't posted the pictures of the ports anywhere yet because I don't have a solid quote from our machinist to determine the feasibility of offering the clearly visible part.

Does that help?

. . . huh?

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