930 JM Pro jams/won't cycle slugs or buckshot

[fastshooter03]

That was one of the questions I forgot to post: how heavy are the buckshot and slugs?

I just assumed they were not LR.

One of the other things I wanted to play with but haven't yet is making different length action tubes.

I was wondering what would happen if you made a shorter tube(spring-ramrod pc-tube-gas piston) and see if it would delay/soften recoil.

I did try running a Remington action spring as it is the same diameter but just slightly heavier.

It worked for most loads but I ran into issues cycling low recoil slugs so I went back to a stock Mossberg spring.

One of the mods I did that definitely lowered pressures was to lengthen the forcing cone at the end of the chamber.

I used a tool I got from brownells and it took some time to get it cut all the way down the taper of the reamer.

How are the barrels on those JM Pros???

It seems a lot of the 930 barrels have non-concentric and/or parallel choke tube threads as well as bending issues.

You can look down the bore of mine and see where it starts bending upward and looking futher you can see the back edge of the choke tube on the left side.

It does not change as you unscrew the choke tube so it's definitely the barrel threads.

I found this out last year when I was doing the barrel reaming and polishing.

Explains why it shoots further right when you use a tighter choke tube and why shot and slugs don't shoot same POA.

Nick

[JimmyZip]

That was one of the questions I forgot to post: how heavy are the buckshot and slugs?

I just assumed they were not LR.

One of the other things I wanted to play with but haven't yet is making different length action tubes.

I was wondering what would happen if you made a shorter tube(spring-ramrod pc-tube-gas piston) and see if it would delay/soften recoil.

I did try running a Remington action spring as it is the same diameter but just slightly heavier.

It worked for most loads but I ran into issues cycling low recoil slugs so I went back to a stock Mossberg spring.

One of the mods I did that definitely lowered pressures was to lengthen the forcing cone at the end of the chamber.

I used a tool I got from brownells and it took some time to get it cut all the way down the taper of the reamer.

How are the barrels on those JM Pros???

It seems a lot of the 930 barrels have non-concentric and/or parallel choke tube threads as well as bending issues.

You can look down the bore of mine and see where it starts bending upward and looking futher you can see the back edge of the choke tube on the left side.

It does not change as you unscrew the choke tube so it's definitely the barrel threads.

I found this out last year when I was doing the barrel reaming and polishing.

Explains why it shoots further right when you use a tighter choke tube and why shot and slugs don't shoot same POA.

Nick

Can you have the barrel cut and re-threaded ?

[fastshooter03]

Yes, in the early days of the 930 the barrel selection was small so a fellow shooter cut off a turkey(ported barrel) and had a thin wall choke tube system cut in.

The only problem with that is the high price tag.

If you are lucky your stock barrel is cut wrong perfectly vertical and you only have an up-down issue.

I basically have to stay with looser choke tubes especially when slugs are involved.

If I get around to making more choke tube sizes for myself, I will try to open up the mouth of the tube so there isn't a step.

Hopefully Mossberg has/can improve their manufacturing process to eliminate this.

Being a machinist I would love to see how it's done in their facility.

Nick

[mpom]

Nick,

How can you tell that lengthening the forcing cone lowered the pressure, and is there a benefit in doing this in a JM930 that does not have issues similar to yours?

Thanks,

Mark

[bullpup]

That was one of the questions I forgot to post: how heavy are the buckshot and slugs?

I just assumed they were not LR.

All of the slugs and buck that I am having problems with are full power slugs and buck. They are what I had on had because they are what I run in my pump gun.

[DaveTimm]

We had this same issue with only full power rounds, even some turkey loads would do it. I found it has more to do with the piston, as well as not having a very solid stance. With a very very solid stance, or when we used a frame post from the shooting shelter as a test, it would cycle fine. It seems this gun is more picky with full power ammo then lower. It seems the high velocity cycle/impulse jams the carrier. I'm still playing with mine every now and then, hopefully one day ill have the ahh ha moment.

[fastshooter03]

Nick,

How can you tell that lengthening the forcing cone lowered the pressure, and is there a benefit in doing this in a JM930 that does not have issues similar to yours?

Thanks,

Mark

Before and after chronographing showed a drop in velocity. I don't remember the exact values though.

The last couple years I did a lot of loading to try and get a slow burning shotgun powder to work at lower velocities(1250-1300 1oz 1100-1250 1-1/8 1-1/4 oz) without leaving a TON of residue in the barrel and action.

I had some really fast low recoil impulse(felt) loads that ran great but filld the barrel and action up quickly with unburnt powder.

After the 2011 Ozarks match I couldn't believe the thing still ran as unburnt powder was falling out the trigger

Lengthening the forcing cone sorta made it worse.

My 930 turkey has a huge X-Rail and mercury weights so it feels a little different than a normal limited gun.

There's no guarantee lengthening the FC will have a benefit that you'll notice.

It's claimed:

shot pattern is less deformd(tighter)

recoil not as sharp

I'm sure those are true but whether or not you'll notice at a match I'm not sure.

Somebody who shoots limited and has had this done would have to chime in here.

Nick

[bobbo5]

Most likely the jamming problem with slugs is caused by a bent lifter. This could have happened at any time. C-Rums is not at fault however. Take your lifter out of the trigger group and lay it on a trully flat surface on one side then the other and you will probably see that it is out of straight. A bit of tweaking with two pairs of pliers will likely resolve the issue.

Another thing that bugs me (probably just an anoyance but could be the cause of the problem) is how loosly the trigger group sits in the receiver. Mine is a Rhythm, but still basicaly the same as JM. If anyone finds a good solution to that problem I would be interested to know.

Good Luck,

Bob

[mpom]

Bugged me too, Bob. Used a feeler gauge set to measure gap between housing and receiver, cut small tabs 1 thousandth thinner and jb welded to trigger housing, one at front and back, both on same side. Play is gone. I posted a picture in one of the jm930 threads. My contribution to red neck gunsmithing.

Mark

[bullpup]

Most likely the jamming problem with slugs is caused by a bent lifter. This could have happened at any time. C-Rums is not at fault however. Take your lifter out of the trigger group and lay it on a trully flat surface on one side then the other and you will probably see that it is out of straight. A bit of tweaking with two pairs of pliers will likely resolve the issue.

Another thing that bugs me (probably just an anoyance but could be the cause of the problem) is how loosly the trigger group sits in the receiver. Mine is a Rhythm, but still basicaly the same as JM. If anyone finds a good solution to that problem I would be interested to know.

Good Luck,

Bob

I'm interested in this. Could you please post a pic of what you mean. Do you mean that the sides of the lifter would not be completely flat against a flat surface like a table. The gun runs like a top with birdshot and low recoil buckshot. I've been busy with wrestling so I have gotten a chance to shoot it with some slugs. Thanks again for all of the help.

BP

[bobbo5]

The part of the lifter that is likely culprit is the two ears at the rear (where they reach around the trigger group body). These lifters are WAY to flimsy and should have been made of heavier guage steel (check out a benelli lifter). The two "ears" can get bent pretty easily, and when the lifter is out of allignment things just don't work quite right.

I see you are from Colorado, will you be at the noveske match in April? I could help you at the match if you want.

Bob

[bullpup]

The part of the lifter that is likely culprit is the two ears at the rear (where they reach around the trigger group body). These lifters are WAY to flimsy and should have been made of heavier guage steel (check out a benelli lifter). The two "ears" can get bent pretty easily, and when the lifter is out of allignment things just don't work quite right.

I see you are from Colorado, will you be at the noveske match in April? I could help you at the match if you want.

Bob

Yes, I am going to be shooting at the Noveske match. That's actually why I was trying to get this all worked out because this will be the first match that I'm going to use this gun at, hopefully. Any and all help would be greatly appreciated!

Also I took it out again yesterday. I tried two more types of full power slugs and it still causes the same type of jam. Even if I just load one shell in the chamber and shoot, it still jams just like I've shown in the pictures. It even does it with Estate handy cap loads. I finally ordered some low recoil slugs yesterday so I'm crossing my fingures that they will at least work for the Noveske match.

What squad are you shooting on? I'm shooting the afternoons on squad 8. Thanks again!

BP

[T3P_Guy]

Bullpup,

One other thing to check that I was also having problems with was when I would push the bolt release all the way down it would sort of click. I found that the front spring got a little out of place as it isn't retained where it contacts the rear part of the shell stop and would cause some binding.

My 930 is the one what was having the issues that Bob was talking about, when I got it the previous owner had the same problems that you guys are having with full power slugs. It eventually started having the same problem with birdshot also.

When I got it I went through the whole thing and cleaned it, polished up any wear areas and took care of anything that seemed to cause any binding.

I saw that the gas system return spring was causing marring and scratching the crap out of the bottom side of the mag tube so I rebent the end of that, cleaned it up and polised the gouges out of the mag tube.

Then when pushing the pusher assembly against the receiver against the return spring I found that it would slightly bind up on the spring as the spring was a little too large in diameter so I retwisted the spring at that point so it would fit tighter around the mag tube. At this time I also put a taper on the inside of the pusher assembly where the spring enters it to help guide it and to avoid the binding.

I then focus'd on the shell stop assembly and I put a bevel on the rear shell catch at the top so if the lifter was able to make it on top of the shell catch the bolt would be able to push it past and back to the down position. I also polished the front shell catch that retains the shells in the mag tube knocking off any sharp edges that contact the shell. I also put a round profile cut in the side of the catch which made loading seem easier and now the shell releases just a little bit sooner. Now instead of the shell catch looking like | it now looks more like ( .

The lifter arms were very bent as Bob was saying, they were twisted and not parallel to each other. I have wear marks on the right side of my trigger housing where it was rubbing for a long time and when pushing it up by hand you could feel it dragging compared to his Rythym so we had to do some tweaking to get it back into shape. The metal is really quite soft and it is very easy to bend so be careful.

The other thing to watch is to make sure that the piston is clean and properly assembled. I had some problems with the bolt not quite going into battery, when I pulled the bolt back I could see the two pins of the pusher sticking into the receiver and when I took the forearm off i could see that the piston was bound up where it enters into the loop under the barrel. I pulled it out and pulled the seal apart on the piston and it had chunks of debris in it stopping it from sliding freely in barrel. After a cleaning at the range it seems to be functioning properly now.

Now the only problem my gun is having is once in awhile I get a shell that gets cocked to the side and the front of the shell is hitting the right face of the barrel. I'm not sure if it is just the lifter needing some light adjusting or it if is caused by the radius on the top of the rear shell stop. Otherwise the gun seems to be running fine and I haven't had the bolt lock back or it not feed a shell after grinding, reshaping and tweaking everything I did.

--Bruce

[bullpup]

Update: I got some Fiochii low recoil slugs. The gun runs them just fine. No carrier lift jams. So now at least I'll think I'll be able to shoot the Noveske match. I'm still thinking about sending it in after the match because I don't like being limited on slugs and buckshot. I'd like to be able to run full power slugs and buck in case I can't get any Low Recoil slugs and buck for a match. Anyways, thanks everyone for all the thoughts and ideas. I've cleaned the gun twice now, thoroughly, since I started this thread and I also took the shell stop out and polished it, but still no luck with regular slugs or buck. Love the gun but would definately like to get this fixed. If I send it back I'll let you all know how it comes back. I'm a little worried that Mossberg won't do anything with it though since I did have the lifter welded. Thanks again.

BP