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Old November 13, 2012, 09:42 PM   #1
Single Six
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Never Seen This Before...Have You?

Today I helped my father in-law with sighting in his pump action Mossberg 12 gauge [not sure of the model]. I was seated, and shooting from a makeshift rest. I quickly noticed something fairly disconcerting: At every shot, the bolt would slam all the way to the rear, just as if I'd begun the process of chambering another round. Problem was, at no time during the firing was my support hand grabbing the forearm of the gun! I double-checked to be sure that none of the fingers on my shooting hand were hitting the bolt release latch behind the trigger guard, and of course I made sure the bolt was all the way forward and locked before shooting. To no avail; every shot from that pump action shotgun would slam the bolt to the rear and kick out the empty shell! After 4 shots, I told my father in-law that I didn't think this was safe, and we put it aside. He did say that in all the years he's had that gun, nothing like this has ever happened. Looks like a trip to the gunsmith is in order. Still...has anyone ever seen this before, and what do you suppose the issue is?
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Last edited by Single Six; November 13, 2012 at 09:50 PM.
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Old November 13, 2012, 09:45 PM   #2
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That's normal with the Winchester 1200, it was made that way (not sure why though) but never seen it with a Mossberg and I've prolly got close to 10K rounds through my 500.
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Old November 13, 2012, 10:06 PM   #3
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Not at all correct.....trip to smith is highly advised.

Just a guess not being able to see it; for some reason the bolt lock is not locking to the barrel extension.
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Old November 13, 2012, 11:08 PM   #4
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it is more than likely that the spring on the bolt lock is broken, weak and then again it mite be clogged up with dirt. the bolt lock is inside of the bolt with the firing pin running threw the center of it. the bolt lock is designed to hold the bolt closed by springing up-ward into the barrel latch hole holding it.it is a simple fix but if you aren't familiar with it then yes indeed take it to a qualified gun smith. the worst is you will need to replace the hole bolt which last time i looked at them i think they were around 20 bucks. good luck and report back with us. maybe we all will learn something new!!
Scott

p.s. there is a great video on you tube explaining how to disassemble a Mossberg 500 you might want to look into that before you go spend money. if you do you will see what i mean about the bolt assembly i had one that was starting to stick so i soaked it ...it was filthy and it works so much better. one more thing DON'T TRY TO DISASSEMBLE THE BOLT if soaking it and cleaning it and oiling it ...take it to shop
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Old November 14, 2012, 12:18 AM   #5
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Scott: I'm not even remotely tempted to try fixing this myself! I know a gunsmith that teaches a gunsmith course at our local community college. He's done some work on my father in-law's rifle before; and if we let him use the recalcitrant shotgun in the classroom for instruction purposes, he won't charge for the repair. I'll get it to him, and when it's done, I'll post here in this thread about what happens.
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Old November 14, 2012, 12:58 AM   #6
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the smartest thing i have hear all day!!!
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Old November 14, 2012, 01:39 AM   #7
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Why, thank you, sir....I know when it's time to call in the professionals!
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Old November 14, 2012, 07:01 AM   #8
Virginian-in-LA
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All well broken in smooth operating pump guns SHOULD do this. Its just like an inertia gun with no spring to return the bolt to battery. You pull the trigger and the hammer drops, which unlocks the action, but the bolt does not unlock until the fore end moves to the rear. The recoil throws the whole gun back, fore end, bolt, and all, and you stop the receiver. I can shoot my Wingmaster one handed and it will do this every time. I cannot believe how this question keeps popping up on gun forums, and the uninformed always say run to the gunsmith.
It usually pops up every year when folks are sighting in their slug guns from a rest, not standing and shooting normally, too.
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Old November 14, 2012, 07:09 AM   #9
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Virginian, thanks much for the input. My 870 has over a thousand rounds through it, is quite broken-in and smooth-running, but it's never done as the described Mossberg has. I guess I'm in that "uniformed" category you mentioned.
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Old November 14, 2012, 08:24 AM   #10
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Well............maybe.

If you're benchresting a pump shotgun with the bare/unsupported forend resting on the front rest, the body's natural reaction to the recoil of a fired shot (forward/against) would be plenty of travel to run the barrel/action forward past the stationary/planted forend (extracting & ejecting), driven by the shooter's shoulder - then when the recoil's over & the body returns backwards to normal position (pulled back by the shooter's tightly-grasping trigger hand), dragging the barrel/action with it, the next round would then be lifted & chambered as the gun passes over the still stationary forend.

All of the above can easily occur in those few tenth's of a second of recoil duration.

Shotguns, especially pumpguns, should not be benchrested, w/o a human body part between any part of the gun & a rest.



.
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Old November 14, 2012, 08:27 AM   #11
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To double check the lock-up on the Mossberg insert a dowel rod into the muzzle (gun unloaded) and the action closed. Press on the dowel rod and pull the trigger. The action will stay locked up untill the forearm is moved back enough to unlock the bolt.
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Old November 14, 2012, 08:45 AM   #12
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"every shot from that pump action shotgun would slam the bolt to the rear and kick out the empty shell!"

Virginian, are you actually saying that this is normal?!?!?!?!?!?!?!?!?!?!?!?!?
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Old November 14, 2012, 09:08 AM   #13
Brian Pfleuger
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Quote:
Originally Posted by PetahW View Post
Well............maybe.

If you're benchresting a pump shotgun with the bare/unsupported forend resting on the front rest, the body's natural reaction to the recoil of a fired shot (forward/against) would be plenty of travel to run the barrel/action forward past the stationary/planted forend (extracting & ejecting), driven by the shooter's shoulder - then when the recoil's over & the body returns backwards to normal position (pulled back by the shooter's tightly-grasping trigger hand), dragging the barrel/action with it, the next round would then be lifted & chambered as the gun passes over the still stationary forend.

All of the above can easily occur in those few tenth's of a second of recoil duration.

Shotguns, especially pumpguns, should not be benchrested, w/o a human body part between any part of the gun & a rest.



.
I must say, I've never seen that come close to happening. I shot an 870 off a rest before every deer season for years. I never had the action unlock.

Physics would seem to dictate just the opposite, in fact. Since the forearm is steady and the action is sliding backwards under recoil, it would be the same as holding the action and pushing the forearm FORWARD. In order to reload the gun, the shooter would have to go back under recoil and then forward several inches, without the forearm sliding in the rest, and then backwards again. That's certainly not a motion that any normal shooter would accomplish due to recoil.
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Old November 14, 2012, 10:53 AM   #14
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Quote:
All well broken in smooth operating pump guns SHOULD do this.
I have three Mossies.. a 500 12Ga. (NIB Apr. 1990), a 500 20Ga. (NIB Jun. 2006), and an 835 (NIB Dec. 1993). The 500 12Ga. has well over 20k documented rounds through it; it must not be broken in yet as NONE of my guns do as described. My brothers Maverick bought in 1992 does not do that, neither do any of my friends 500's or 870's nor any of the ones I have shot over the past 25+ years.

Slamming the bolt to the rear and ejecting the shell suggests to me that the OP has it looked at by someone in the know. His chosen course of action is not only wise but commendable IMHO because if there really is a problem as some of use believe, the next generation of smiths will have an example to diagnose and correct in an expert setting.
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Old November 14, 2012, 01:44 PM   #15
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Quote:
All well broken in smooth operating pump guns SHOULD do this.

yes all shotguns should do this is the same as saying all cars should have anti-lock breaks.

no all shotguns do not do this.
no all cars do not have anti lock breaks!!
yes please have it looked at by a professional.
no don't listen to misleading misinformation!!

i have been a exclusive Mossberg shotgun owner for many many years(this makes me experienced not an expert) tho i have other shotguns Mossberg is my preference. i have never had a Mossberg shotgun do this nor shall i expect to
i am sure mine are all very well broke in and don't feel the need to prove it!!
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Old November 14, 2012, 03:23 PM   #16
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The Virginian, is correct. The first motion of a fired pump gun, recoil sends the gun back, the forearm is in your hand or resting on something, this causing the forearm to be pushed forward, which unlocks the bolt. Now with about 7 lbs of gun moving toward your shoulder with say 30 foot pounds of energy, you stop that motion, except for the bolt which was unlocked when the forearm was pushed forward or more accurately left behind, the bolt continues aft ejecting the shell. Virginian in LA is exactly correct. If the OP or his father in law would shoot the gun moving objects they would not have noticed the inertia since the gun would not be arrested as quickly.

Single Six, The standard gunsmith test for pump guns involves, in a safe direction, pull the trigger on an EMPTY gun while holding the forearm firmly back. If all is well the forearm will not come back until you nudge it forward then pull back. If the mossberg passes this test there is nothing wrong with it.

Last edited by birdshot; November 14, 2012 at 03:41 PM.
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Old November 14, 2012, 05:45 PM   #17
jaguarxk120
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OK the gun opens when fired from the bench.

The big question I have is how does it work when fired from the shoulder????
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Old November 14, 2012, 07:51 PM   #18
SHR970
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Quote:
birdshot said: The Virginian, is correct. The first motion of a fired pump gun, recoil sends the gun back, the forearm is in your hand or resting on something, this causing the forearm to be pushed forward, which unlocks the bolt. Now with about 7 lbs of gun moving toward your shoulder with say 30 foot pounds of energy, you stop that motion, except for the bolt which was unlocked when the forearm was pushed forward or more accurately left behind, the bolt continues aft ejecting the shell.
I just took my 22 yo. 500 and after simulated firing on a snap cap and dropped it from a measured distance of 24". 5 cycles of fire it and drop it from 24" vertically onto its butt on a hard floor and the bolt would not open all the way let alone eject the round. To get consistent ejection, the gun had to be dropped from 30".

This is a situation based on sudden stop and not a persons body recoiling with the system. An object falling from 24" is moving at 11.35 ft./s. A 1 oz. load fired at 1300 fps in a 7.5 lb. gun gives a recoil velocity of 11 ft. /s. An object falling from 2 1/2 feet is moving at 12.7 ft./s. To achieve 12.7 ft. /s you need a 1 1/8 oz load at 1300 fps in a 7.5 lb gun. Again 12.7 fps was the minimum to get the snap cap to consistently come out of the gun and land next to it. The OP didn't detail the shells he was using however we have baseline figures to work with here.


If a broken in Mossberg barely performs as described by the OP in a sudden vertical stop from 2 1/2 feet how is it supposed to do it during a more attenuated stop horizontally? In a horizontal stop gravity is not continuously acting on the bolt / slide assembly in a manner that will be conducive to opening it; vertically gravity is still trying to accelerate the freed assembly down. On the bench the persons body absorbs some of the free recoil energy and the rate of deceleration is seriously attenuated; a drop onto a hard surface gives a much more rapid deceleration.

Still sounds like a good time to have it looked over. If nothing else, it is a free check up anyhow.
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Old November 14, 2012, 08:07 PM   #19
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I can also point my Wingmaster at the ceiling and push the fore end release and the action will drop almost all the way open. On my last one it would drop all the way open. I guess Wingmasters are a lot smoother than Mossbergs, sorry - who knew.
And the standard test is either pulling back on the fore end or pushing a dowel down the barrel of an unloaded gun and pulling the trigger and seeing if the bolt stays locked.
You will also note I said I could do it firing my Wingmaster one handed, i.e. nothing restricting the fore end.
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Old November 14, 2012, 08:44 PM   #20
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Jaguar: Good question, for which I admittedly have no answer. We quit firing after the 4th round; Pop was a bit wary of continuing. The gun will receive a gunsmith going-over before another load goes down that barrel.
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Old November 14, 2012, 08:58 PM   #21
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SHR I am sure your guns are in good working order. But you really don't have a firm understanding of how a pump gun works. The forearm needs to move forward before it can move backward. The trigger does not unlock the bolt. In your experiment it sounds as the bolt is unlocking during the rebound.
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Old November 14, 2012, 09:44 PM   #22
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I am very familiar with the internals of the model 500 and how they work. I am also familiar with the usual suspect issues that the model has from time to time and their cures. All you need to do with a Mossy is to not have rearward pressure on the action and it will unlock; ie pull trigger while not holding forearm. They do not need the bolt or forearm to go forward. Take one and try it for yourself. Not all designs operate exactly the same way. If the OP had said Win. 1300 or old school Ithaca 37 I would have a different answer to his question but he said Mossberg 500.

I can unlock the bolt and hold all of mine upside down and they will not cycle open under their own weight. Both 12 gauge Mossy's of mine have quite a few rounds through them and don't come close to exhibiting the behavior of the OP's gun.

With that I am done discussing this.
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Old November 14, 2012, 09:50 PM   #23
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The force of the recoil is ejecting the shell because you aren't holding the action closed when shooting from a rest..simply Pulling the trigger and firing is disengaging the bolt lock. This is because the gun is well broken in. This is normal. If he was chambering a round, not firing, and pulling the forend back and it opened, this would not be normal.

Wow @ this thread


This happens on Winchester 1300s, 870s AND Mossbergs...I've owned at least 3 or 4 of all makes. If the trigger being pulled didn't disengage the bolt lock, then what does???? What other manual operation is involved other than firing and racking the slide? Why does holding the slide release down and racking the gun allow the bolt to unlock and slam rearward ejecting a shell, spent or otherwise? Think a little bit here, please. LOL

This very occurance is actually a measuring stick for me for a well broken in pump gun.

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Old November 14, 2012, 10:16 PM   #24
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Apology to SHR970

I was wrong. You are correct the mossberg does not need forward movement of the forearm. I looked at a blueprint for the 500 and see that the hammer stricking the firing pin provides the inertia to unlock the action. Once the hammer strike disengages the lock, the bolt is free to move to the rear.
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Old November 14, 2012, 10:33 PM   #25
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OO take the gun to the shop
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