Jump to content
Brian Enos's Forums... Maku mozo!

How to tune a 930 JM Pro to get it to run.


Tuflehundon

Recommended Posts

I have had my Mossberg JM Pro for a couple of years now and love the gun. You hear good and bad things about it, but most people don't seem to know what needs to be done to the the gun running. So I figured I'd post up what I have learned. Not so much wanting to get into the go faster mods with this post. Mainly want to help people who are having issues with the 930's.

There are 3 things you need to do to get a JM Pro running right. They are all very simple to do, and can be done by just about anyone if you have a few basic tools. (this is assuming something isn't broken on the gun)

1. 1st thing you should do is throw away the follower that comes with the JM Pro and get something like the GG&G follower. The stock follower works great until you add an extension to the tube. Then it sucks. It will get caught up on the edge were the 2 tubes meet as it will always be a slight difference in size. The GG&G follower has a beveled front edge that keeps it from catching. The 1st 3 or 4 rounds will feed fine (depending on if you have the 22" or 24" gun), then it will seem as if there is no pressure on the shells. This is because the stock follower is caught up at the junction.

The newer guns have a different follower than mine did, but it still sucks.

2. Grind flat the front end of the recoil spring that is in your stock. I used a dremel with a grinding wheel to grind it flat. This will prevent it from digging into you recoil spring plunger which it can do very quickly. If you have burrs on your plunger, then you should also round off the rear edge of it. I put the plunger in my drill, turned it and and ran it against a file to round it off. These burrs can catch on the sides of the recoil spring tube and slow the gun down, causing misfeeds, failure to feeds, failure to eject, etc. My gun ran great until about 1200 rounds then started having big issues. This was the cause of most of my problems. This can also happen very quickly with the 930's. Helped a friend tune his gun last month that was new. he had less then 200 rounds through it and his plunger was starting to burr already. My gun just seemed like it ran slow. Here is a slow motion video of my gun when it started acting up.

https://www.youtube.com/watch?v=ajZHs4GKKlI&list=UUddrzgDitsWidjpwTUZmCiw

Here is a pic of what my recoil spring plunger looked like after 1200 rounds.

1375890_10204985311574810_83636874848748

I was also having the next issue, which is the feed from the tube problem, but wasn't able to diagnose it till I fixed the recoil spring plunger.

3. Polish the front edge of the carrier latch. This is the part that releases shells from the tube as the gun cycles, or you press the release. It can be sharp, and will catch on the ammo and cause the gun not to feed. If you are having problem getting the 1st round to 2 to feed from the tube when it's full, you are having this problem. Took me 10 minutes to round the edges off with a flat file. This can happen with some guns out of the box, or like mine after 1200 rounds. It doesn't happen with all guns, but it's such an easy fix that I would do this anyways. Brownells has a good tutorial on it here:

http://www.brownells.com/.aspx/lid=1583 ... be-Problem

95% of the problems I have seen with these guns over the past couple of years has had to do with 1 of these things, or a combination of them.

Link to comment
Share on other sites

Great write up. I still haven't had much issues with my JM Pro. I do believe I have around 1,500 rds thorough it. I never gave much attention to my recoil spring plunger or the spring. I'm going to have to take a look at mine.

Another note to all JM Pro shooters is the piston. It gets caked up with carbon fairly quickly so that needs to be cleaned very well. And also light primer strikes can happen with the JM. Drop in the trigger spring from 10/22 and that will fix the light primer strikes as well.

Link to comment
Share on other sites

Great write up. I still haven't had much issues with my JM Pro. I do believe I have around 1,500 rds thorough it. I never gave much attention to my recoil spring plunger or the spring. I'm going to have to take a look at mine.

Another note to all JM Pro shooters is the piston. It gets caked up with carbon fairly quickly so that needs to be cleaned very well. And also light primer strikes can happen with the JM. Drop in the trigger spring from 10/22 and that will fix the light primer strikes as well.

For the piston issues fangalang mentioned, the gun just needs to be cleaned properly. The piston is 3 parts, many people don't know that. Jerry Miculek has a great video showing you how to clean it right. He starts talking about the piston at about 7:40 in this video.

Link to comment
Share on other sites

Get yourself the wire brush that Jerry uses in that video. I got one from Brownells. It says it's for the Remington 11-87 but it fits perfect in the piston tube on the 930 barrel. This is a common place for carbon to build up and prevent the piston from going all the way in. This causes the bolt not to lock up and the trigger will not function. I found this out the hard way. Just a few turns with the wire brush and the carbon is gone.

Sent from my iPad using Tapatalk

Link to comment
Share on other sites

Keeping the piston clean is the #1 priority in these guns. I ran a JM PRO for about a year and never had any problems with mine, but I saw one after another lay down at matches, and talking to the owners almost always resulted in the gun not being clean enough, or the weak factory magazine spring.

Keep it clean and replace the factory magazine spring with a Nordic.

Link to comment
Share on other sites

I'll also add that there are likely several followers out there that will work fine in the JM Pro. As long as the have a decent beveled edge at the front they should work. The GG&G is the only 1 I tried in mine. It worked so well I never tried any other followers.

Link to comment
Share on other sites

Great post! Had not checked the recoil spring, but now will do so. My 930 ran great for 250 rounds and then got really erratic after cleaning it. Like the OP, the mag tube and follower were contributing factors.

I also replaced with a GG&G follower which improved feeding. Only other note on magazine tuning was that I beveled/polished the interior edges of the mag tube and extension which also helped with consistent feeding. Finally, I did not fully tighten the mag tube extension, but rather, left it about a quarter turn loose and then tightened the clamp. Read a previous post here about concentricity of these tubes causing shells to hang up.

Thanks for the additional info!

Link to comment
Share on other sites

  • 3 weeks later...

WOOO! thank you for this post. I have been looking into getting my first 3G specific shotgun and was leaning to the 930 (the price is awesome). read a bunch of posts and I got scared. I think yall brought my hope back for being able to run some matches this summer.

Link to comment
Share on other sites

For anyone considering a 930JM (especially if you have a limited 3gun budget) -

Everyone who posted about keeping the piston etc clean are 100% correct !

I've owned (2) Benellis, a 930JM, a Berreta 1301 and now have a 2nd new 930JM - ALL are great SGs but if you're on a "budget" you can't beat the 930.

My current one has the only the lifter welded (CRums) the loading port opened up, stock mag tube & follower sanded and smoothed, and a brighter FO sight.

It'll run 400-450+ of any type 2-3/4" shells 100%. And after a bit of practice load2 is smooth and EZ.

Don't hesitate to get a 930JM if you're not wanting to spend $12-1500 or are a less than gung ho serious 3 gunner ...........................

:D:D:D

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...