I had my Mossberg 500 out at the range last week and I was running some various loads through it to try out my new Knoxx SpecOps stock.
I tried some S&B “00” 3” buck and my bolt was part way open after the shot.

I figured I partially pumped it after firing (not being familiar with the different recoil handling with the new stock) so I tired a shot one handed (so I wouldn’t partially work the pump with my weak hand) and it left the bolt partially open again.

WTF?  Does anyone know what might be going on?

Does the SpecOps stock have the built in shock absorber?

Odds are pretty good that unless your bolt locking lug is sheared off, the gun is fine, and it's just opening do to a little inertia.  My old Maverick 88 used to do that, particularly with heavy loads and a loose grip.

You were probably pulling back a little on the forearm when you fired the weapon.  When you pulled the trigger and the action released, then that, coupled with recoil, partially opened to bolt.  If the bolt was unlocked when the weapon fired, then you would certainly know it, or your widow would anyhow.

It is quite normal for the action to unlock after the shotgun is fired in a Pump Action shotgun.  Just be safe make sure the action locks on empty and a loaded chamber.  If in doubt take it to a gunsmith for a check over.

its the super secret Mossberg assisted pump action dont worry

Quoted: You were probably pulling back a little on the forearm when you fired the weapon.  When you pulled the trigger and the action released, then that, coupled with recoil, partially opened to bolt.  If the bolt was unlocked when the weapon fired, then you would certainly know it, or your widow would anyhow.

I wondered about that but I tried another shot one handed to be sure I wasn't doing it with my weak hand and it did the same thing.  I can't figure out WTF.

Quoted: its the super secret Mossberg assisted pump action dont worry

OK, super-secret-Mossberg assisted pump doesn't make me feel good.

Quoted: Does the SpecOps stock have the built in shock absorber? Odds are pretty good that unless your bolt locking lug is sheared off, the gun is fine, and it's just opening do to a little inertia.  My old Maverick 88 used to do that, particularly with heavy loads and a loose grip.

Yes, that's exactly what it is...a recoil compensating stock with a built in shock absorbing system.
Thanks for the 411.

BTW, I'm from Canada too.  Cheers Canuck!

I had a Remington Super Slug shotgun that would do the same thing when firing the Winchester Partition Gold sabot slugs. The bolt would be about half open after the shot. I attributed it to the recoil pulling it back after it unlocked.

Quoted: Quoted: its the super secret Mossberg assisted pump action dont worry OK, super-secret-Mossberg assisted pump doesn't make me feel good.

My 870P does the same thing...more so it seems with heavier loads.  It's no big deal as long as the weapon cannot fire with the bolt unlocked.  Try this: after checking to make sure the weapon is unloaded, cock the action, then, while pulling back on the forend, pull the trigger.  The weapon should "fire" and immediately, the forend should release and the tension you are placing on it will pull it to the rear.  

Quoted: Quoted: Quoted: its the super secret Mossberg assisted pump action dont worry OK, super-secret-Mossberg assisted pump doesn't make me feel good. My 870P does the same thing...more so it seems with heavier loads.  It's no big deal as long as the weapon cannot fire with the bolt unlocked.  Try this: after checking to make sure the weapon is unloaded, cock the action, then, while pulling back on the forend, pull the trigger.  The weapon should "fire" and immediately, the forend should release and the tension you are placing on it will pull it to the rear.

Thanks, I'll check it out like that.

If it can't fire out of battery I'll be happy.  In that case I guess I'll just have to adapt my pumping rhythm (which I have to relearn anyway with the new stock).

Yes, the bolt unlocks when you pull the trigger.  If the recoil is stiff, it will push the action open.  In the picture below is a Mossberg 500 8-shot just nanoseconds after the trigger was pulled.  The ammo was Winchester 2 3/4" 1oz foster-style slugs.  Notice how the action is almost all the way open and the shell almost auto ejected


This is completely normal.  That particular gun is a decade and a half old and has had thousands of rounds fired through it.  The more you shoot them, the smoother they get, and the more likely that the action will do this.  It is nothing to worry about at all.

2guntom
2guntom.com
454 Casull +
AntiAmerican
Destruction

Quoted: Yes, the bolt unlocks when you pull the trigger.  If the recoil is stiff, it will push the action open.  In the picture below is a Mossberg 500 8-shot just nanoseconds after the trigger was pulled.  The ammo was Winchester 2 3/4" 1oz foster-style slugs.  Notice how the action is almost all the way open and the shell almost auto ejected www.2guntom.com/454/group/Im002847wf2.jpg This is completely normal.  That particular gun is a decade and a half old and has had thousands of rounds fired through it.  The more you shoot them, the smoother they get, and the more likely that the action will do this.  It is nothing to worry about at all. 2guntomwww.2guntom.com/454/group/2gunsfiring_v1.gif 2guntom.com 454 Casull + AntiAmerican Destruction

OK, thanks.  Mine is getting to where it is "broken in" so I guess like you say it's getting a little smoother.

Yep, the more you shoot them, the smoother the action gets.  It is just part of the "break-in" cycle.

At a local gun store is a High Standard pump shotgun.  Somebody sent it to the local 'smith that specializes in refinishing.  When he Dura-Coted the gun, he did it inside and out.  You talk about a slick action!  When you hold the gun muzzle up and hit the slide release, gravity pulls the action completely open!  They don't get any slicker than that!

2guntom
2guntom.com
454 Casull +
AntiAmerican
Destruction

Thats in the Frequently asked question section on Mossbergs website.

they say its normal, and it usually happens when firing off a rest or supported position.

Quoted: Thats in the Frequently asked question section on Mossbergs website. they say its normal, and it usually happens when firing off a rest or supported position.

Thanks, I didn't even check there.  I am so conditioned by ARFCOMs collective wisdom I come here to check any and all knowledge rather than going to a manufacturers site.

Definately normal. My Mossy 835 does it, especially with those shoulder-separating turkey loads.

Quoted: WTF?  Does anyone know what might be going on?

All pump guns do this.  Some heavy loads will damn near eject the empty for you, especially in 2 3/4" magnums.  Learn to use it to your advantage and your follow up shots will be even quicker.

Use the force Luke, don't over think it.

PS don't worry about slam fire either, your gun won't do it.

Quoted: Quoted: WTF?  Does anyone know what might be going on? All pump guns do this.  Some heavy loads will damn near eject the empty for you, especially in 2 3/4" magnums.  Learn to use it to your advantage and your follow up shots will be even quicker. Use the force Luke, don't over think it. PS don't worry about slam fire either, your gun won't do it.

I'm cool with that.  I just wanted to be sure it wasn't some sign of impending catastrophic failure.

I have a 500 that has about 5000 rounds through it..I think I have only cleaned her three times (I can not remember when I did clean her last), I have 'yet' to have this happen to mine.

I had a rotary bolt Winchester 1200 pump that would do that also. The funny thing is I remember an article years ago in Guns & Ammo or American Hunter that explained how one could use this feature to one's advantage for fast follow up shots. Actually it was many, many years ago now that I think about it.
Someone can correct me if I am wrong but the majority of the pump action shotguns have a disconnector of some sort in the trigger assembly that hold the action closed when cocked and locked. After the trigger is pulled, the bolt locking part of the trigger group is disengaged allowing inertia to open the breach. By this time your shot string is well downrange. The Mossberg 500 has a rectangular piece of steel that hinges upward and sits behind the bolt holding it closed when cocked and locked. When one pulls the trigger this piece of steel drops down to release the bolt to cycle. I discovered this piece when a 500 I purchased from a pawn shop would NOT lock up when it was cocked and when you fired it the bolt WOULD lock up where you had to hit the release button behind the trigger guard. That is exactly the opposite of how it's supposed to work and probably why it ended up in a pawn shop. It turned out that the little rectangular locking piece appeared to be stamped metal and was a hair longer than spec which prevented it from hinging up and locking behind the bolt. When firing the weapon, the inertia allowed enough slack for that locking piece to ride the recoil up against the back of the bolt thereby locking the action AFTER it was fired. I removed the trigger assembly and removed that extra hair of metal off the end of that piece and problem solved. BTW that was 10 years ago (how time flies) and I've never had any problems with that 500 again. Sorry to be so wordy but if it happened once it can happen again.

Quoted: I had a rotary bolt Winchester 1200 pump that would do that also. The funny thing is I remember an article years ago in Guns & Ammo or American Hunter that explained how one could use this feature to one's advantage for fast follow up shots. Actually it was many, many years ago now that I think about it. Someone can correct me if I am wrong but the majority of the pump action shotguns have a disconnector of some sort in the trigger assembly that hold the action closed when cocked and locked. After the trigger is pulled, the bolt locking part of the trigger group is disengaged allowing inertia to open the breach. By this time your shot string is well downrange. The Mossberg 500 has a rectangular piece of steel that hinges upward and sits behind the bolt holding it closed when cocked and locked. When one pulls the trigger this piece of steel drops down to release the bolt to cycle. I discovered this piece when a 500 I purchased from a pawn shop would NOT lock up when it was cocked and when you fired it the bolt WOULD lock up where you had to hit the release button behind the trigger guard. That is exactly the opposite of how it's supposed to work and probably why it ended up in a pawn shop. It turned out that the little rectangular locking piece appeared to be stamped metal and was a hair longer than spec which prevented it from hinging up and locking behind the bolt. When firing the weapon, the inertia allowed enough slack for that locking piece to ride the recoil up against the back of the bolt thereby locking the action AFTER it was fired. I removed the trigger assembly and removed that extra hair of metal off the end of that piece and problem solved. BTW that was 10 years ago (how time flies) and I've never had any problems with that 500 again. Sorry to be so wordy but if it happened once it can happen again.

Interesting.
Then like Canuck said it sounds like it could have to do with inertia and the recoil compensating stock.  Because with the new stock the shotgun actually moves back towards me as recoil is absorbed by the stock then it comes to a stop at the end.  By that point the disconnect would have come into play so when the action, bbl etc are moving back and suddenly stop the pump (and bolt I guess) must still have enough momentum to partially open the action.

I think that's probably what is going on. I don't think at this point there is a problem with your shotgun. If you wish you can get Mossberg's # from their website and give their tech dept. a call. They have always been very helpful to me the few times I have called.

http://www.knoxx.com/NewStyleKnoxx/Products/COPstock.htm

Watch the video when he shoots one handed.

Quoted: http://www.knoxx.com/NewStyleKnoxx/Products/COPstock.htm Watch the video when he shoots one handed.

Good eye.  Thanks.

Shooting my M500 with a pistolgrip like a regular handgun, with both hands on the grip, gives the same results.  I can't find the pic but I'll post it if I do.  That was with #8 shot low brass.