Bad slide lock on Mossberg 500

**4V50 Gary** · July 28, 2014, 10:56 PM

Along with the action lock, the bolt slide keeps the bolt from unlocking. When the hammer is forward, it pushes the action lock down, allowing the bolt slide to be pushed rearward when the forearm is pushed to the rear.

I'm looking at the bolt slide that is rounded in the area where it contacts the action lock. Because of that, it doesn't take much pressure to push the forearm back without even having to depress the action lock. I know the bolt lock does rise to engage the slot in the barrel. When the barrel is removed and the action worked, I can see the bolt lock rise and fall.

Has anyone else out there ever found a defective Mossberg that can be released without depressing the action release?

g.willikers · July 29, 2014, 08:07 AM

Maybe a new action lock lever spring is needed??

Dixie Gunsmithing · July 29, 2014, 08:17 AM

The action slide lock should only have a very minimum rounded corner on it to break a sharp edge. The face that fits against the bolt slide is on a slight taper, but is flat for most of the surface. If it is rounded over, it would be more like a cam and not hold it. The reason for the taper is so it will go up behind the bolt slide easily, and release it easily, but still hold itself under friction. It is similar in action to a bunch of other guns, which have a slight tapering angle to the front face. Take a look at this first, and it's spring.

moss500-act-lock by matneyw, on Flickr

I see nothing rounded over on the action slide, except the front of the two ways.

moss500-slide by matneyw, on Flickr

**4V50 Gary** · July 29, 2014, 08:35 AM

Thanks DixieGunsmithing. The bolt slide's left side is rounded I'm trying to fix. Like I said, hence its failure to stay locked up. I think it's a bad casting.

g.willikers · July 29, 2014, 08:54 AM

While we're talking about funny stuff going on with the 500, I have a question.
On my thirty year old version, the action is almost semi-auto.
It opens and comes back on its own when fired, and only requires forward motion on the part of the shooter to chamber the next round.
I actually like it that way, 'cause it really speeds up the shooting process and has allowed me to win some shotgun side matches against the autoloader guys.
And they thought it was all me.

But if the action isn't consciously held forward and closed when the trigger is pulled, it comes flying back real good.
How far depends on the ammo.
Although, there's no complaint there, either, as pushing the gun forward with the support hand helps with recoil for those nasty slugs.
Kind of weird, though, and I'd be reluctant to let anyone else borrow it.
When the hammer is cocked, though, before pulling the trigger, it acts completely normal - the action will not open unless the action lock lever is depressed, as designed.
Anyone have an idea of what's going on with the old beast?
Just out of curiosity, it would be nice to know what would be required to make it normal in case of trading it off someday.
Doubtful if that would ever happen, as the pump shotgun has to be about the most all around useful shootin' iron ever invented.

**4V50 Gary** · July 29, 2014, 09:16 AM

G. Willikers, bet the action lock lever is worn on yours. Reshaping it with a file may help (or just replacing it with a fresh one).

g.willikers · July 29, 2014, 10:01 AM

There must some intricacy of design that keeps the action closed momentarily for firing, and then allows the action to open to be pumped for the next round, without having to depress the locking lever.
Maybe some balance of leverage, parts interaction, spring pressure or something is involved.
The fact that the release operates as it should when the action is closed on a live round, but allows the action to come flying back under recoil is confusing.
Unless the release is consciously activated to allow the action to open on a live unfired round, the action will absolutely not open.
So far the only thing I can think of, as you say, is to just quit assessing and just change the release and spring and see what happens.
Finding out how these things work enough to disassemble, reassemble and fix the obvious isn't all that difficult.
However, finding the information on how they actually work in intricate detail is very difficult to come by.
Maybe all will be revealed by osmosis if I just stare at it some more.

Dixie Gunsmithing · July 29, 2014, 05:25 PM

The bolt and slide mechanism works similar to a 31 Remington on these 500's, or it is the closest I can think of. The trigger housing, and its works are slightly different.

The front of the action slide lock, which has that slight taper, is supposed to be a steep enough angle that friction will hold it, even when the hammer falls, until recoil will cause the forearm/slide to pull forward slightly, and then follow the gun backwards. When it does this, it is enough to separate the action slide lock from the action slide, and allow it to drop, where the slight taper would let it release.

The problem cones about if the taper gets too slick, or rounded off, and it falls with the hammer, which would allow the whole thing to start to come back under recoil, and unlock the bolt from the barrel. Winchester overcame this, on the model 12, and Mossberg similarly, by placing that 1-turn or so spiral spring on the outside of the magazine tube, which kept pressure on the action slide lock, and only allowed the recoil to slightly pull the forearm forward, while it was being held by the shooter. This allowed some wear to be taken up, and it keep working. I had a model 12 to do this very thing years ago, and the action slide lock was worn out.

One might be able to straighten the angle of the taper out, on the action slide lock, if its not worn too much, and even give the face a shot with a sand blaster to increase the friction. If it's too warn, then filing on it will only make it worse, I'd imagine, as it would be shortening it, and give the action slide too much slack. Mossberg parts are generally cheap enough, that it would be much less expensive to just replace it. Also, see if the spring on the outside of the magazine tube is weak.

**4V50 Gary** · July 29, 2014, 08:35 PM

G. Willikers - Look at that lever posted by Dixie Gunsmith. The left hand side that has that slight angle is what bears against the bolt slide. On the bolt slide, do you see where that cut out is on the right hand side. The left side of the cutout is the bearing surface that the action lock bears against.

When the hammer is cocked, the action lever is pivoted up such that it contacts the bolt slide. When the hammer is dropped, the action lever is pivoted down and allows the bolt slide to move back when the forearm is pumped.

g.willikers · July 29, 2014, 09:30 PM

Yes, it all makes sense, now.
Thanks you guys for the explanation.
I didn't realize that the bolt, slide and lever operated in exactly that fashion.
Thanks again.
With so many folks owning this model, no doubt plenty of others will learn something from this thread, too.

Dixie Gunsmithing · July 29, 2014, 10:03 PM

g.willikers,

A way to check the correct operation is by dry firing the gun. If it is correct, when the hammer falls, the action slide lock should still hold the slide. Then, if you push forward on the forearm, the action slide lock should release, (the button go upwards), and allow it to be pumped. The movement of the forearm is a very slight amount, which is pushing against the spring on the outside of the magazine tube, making the 500 operate just like the model 12. That spring is located behind a ring on the magazine tube, as is covered by the forearm, so the forearm would need removed to get to it.

g.willikers · July 30, 2014, 08:14 AM

Here's an excellent video, by Midwayusa, using a cutaway model of the Winchester 12, showing the inner works and explaining how the spring around the mag tube you described works with the slide lock lever.
http://www.youtube.com/watch?v=_uuMJ9grq-0
Too bad the Mossberg doesn't have it.

**4V50 Gary** · July 30, 2014, 08:41 AM

That's a good video. Thank you. We had to learn about the Win M12 at school. In concept, the Mossberg action lock works the same way. With the hammer back, the slide lock is raised and hold the bolt slide back. With the hammer forward, the slide lock is lowered, allowing the bolt slide and bolt to be moved backward.

Dixie Gunsmithing · July 30, 2014, 11:28 AM

The Winchester model 12 was the brainchild of Thomas C. Johnson. Some give Browning the credit for the removable barrel, but if you look at the patents, it was really a designer with the last name of Mason, I believe, that came up with the idea to use it on Winchester's lever action rifles, and was an older Winchester designer. The part Browning was involved in, was the rear portion of the 1897, and 1893, and using the Mason design for the barrel assembly.

Anyhow, at the time, the model 12 was the most advanced, and better working pump designed, and I think it is until this day. During the model 12's design time, Browning had designed the Remington model 17, which later became the Ithaca 37, along with developing the Auto-5 with Winchester.

Anyhow, Johnson came up with the tilting breech block design, where the action rod from the forearm worked it off a cam on the bolt, and the action slide lock came up behind the action slide rod, to block it, but used this tapered-friction holding design using recoil. The action slide, with the tilting breech block idea, was either a Browning or Remington/Pederson idea, I can't remember which, but the action slide was used in the Remington 17. Remington later used a lifting bolt lock in an no-tilt bolt, using an action slide to actuate it. In any case, I think this is about the first place I remember seeing this action slide in use, on the 17 and in the model 31. The 31 was a really great gun, as the Pederson designed 10 and 29 were bad to break down all the time. Thus, the 31 was the predecessor to the 870, and several other pump guns, like the Mossberg, and I think some JC Higgins and Hi Standards. The 17, only made in 20 ga., was sold to Ithaca, where a law suit with Pederson unfolded, and delayed its manufacturer for several years.

Edit:

For some of you history buffs, here are two of the William Mason patents for Winchester. It shows the use on the 93, but he had some earlier patents on the rifles, when he first came there from Colt in 1882.

US499464-1 by matneyw, on Flickr

W-Mason2 by matneyw, on Flickr

**4V50 Gary** · August 12, 2014, 06:58 PM

I returned the old slide lock, a letter of explanation and my armorer's certificate to Mossberg. Today I received a replacement slide lock and dropped it right it. The bolt locks now and won't go back until the action released is depressed. Success!

Dixie Gunsmithing · August 12, 2014, 07:29 PM

Great! Glad it did.

Earlier, when I mentioned the model 12 I had that did this, I tried fixing it by expanding the metal on the cam end of the lock, and reshaping it. I found it was better, even on the model 12, to buy a new lock. Some give warnings on replacing these, but it is not as bad as is said. You have to re-stake two small straight springs to the new model 12 lock, and simply replace it. Anyhow, I had all kinds of fit problems, including a hang fire that went off at my hip, while trying to fix the old lock, so I always say replace them if available.

OKGunDealer · September 12, 2016, 07:09 PM

I'm a big fan of Mossberg and anything American made. I just bought a model 500 in 410 and wouldn't you know? She won't lock up unless you jam the bolt closed. I checked it straight out of the box and the little lever hangs when the trigger assembly is in the receiver frame. It works fine when the trigger assembly is out of the gun.

I could hear surprise in the lady's voice after she asked how l had my gun. I asked her what time it was? About 11 hours. Instantly she said she'd send a label and take care of the shipping both ways. I'm supposed to get my gun back in two weeks.

My disappointment isn't that the gun was broken right out of the box. Well, maybe a little. I'm more concerned that the cause hasn't been addressed in the several years it's been happening. I'm happy that they agreed right away to make it right. That part I couldn't be happier about.

g.willikers · September 22, 2016, 11:24 AM

No harm in beating a dead horse.
It's supposed to aid in tenderizing them.
This thread is only two years or so old.
You should see some of them.
Good info is always appreciated whenever it was discussed.

July 28, 2014, 10:56 PM	#1
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	Bad slide lock on Mossberg 500 Along with the action lock, the bolt slide keeps the bolt from unlocking. When the hammer is forward, it pushes the action lock down, allowing the bolt slide to be pushed rearward when the forearm is pushed to the rear. I'm looking at the bolt slide that is rounded in the area where it contacts the action lock. Because of that, it doesn't take much pressure to push the forearm back without even having to depress the action lock. I know the bolt lock does rise to engage the slot in the barrel. When the barrel is removed and the action worked, I can see the bolt lock rise and fall. Has anyone else out there ever found a defective Mossberg that can be released without depressing the action release? __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

July 29, 2014, 08:07 AM	#2
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	Maybe a new action lock lever spring is needed?? __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.”

July 29, 2014, 08:17 AM	#3
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	The action slide lock should only have a very minimum rounded corner on it to break a sharp edge. The face that fits against the bolt slide is on a slight taper, but is flat for most of the surface. If it is rounded over, it would be more like a cam and not hold it. The reason for the taper is so it will go up behind the bolt slide easily, and release it easily, but still hold itself under friction. It is similar in action to a bunch of other guns, which have a slight tapering angle to the front face. Take a look at this first, and it's spring. moss500-act-lock by matneyw, on Flickr I see nothing rounded over on the action slide, except the front of the two ways. moss500-slide by matneyw, on Flickr Last edited by Dixie Gunsmithing; July 29, 2014 at 08:33 AM.

July 29, 2014, 08:35 AM	#4
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	Thanks DixieGunsmithing. The bolt slide's left side is rounded I'm trying to fix. Like I said, hence its failure to stay locked up. I think it's a bad casting. __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

July 29, 2014, 08:54 AM	#5
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	While we're talking about funny stuff going on with the 500, I have a question. On my thirty year old version, the action is almost semi-auto. It opens and comes back on its own when fired, and only requires forward motion on the part of the shooter to chamber the next round. I actually like it that way, 'cause it really speeds up the shooting process and has allowed me to win some shotgun side matches against the autoloader guys. And they thought it was all me. But if the action isn't consciously held forward and closed when the trigger is pulled, it comes flying back real good. How far depends on the ammo. Although, there's no complaint there, either, as pushing the gun forward with the support hand helps with recoil for those nasty slugs. Kind of weird, though, and I'd be reluctant to let anyone else borrow it. When the hammer is cocked, though, before pulling the trigger, it acts completely normal - the action will not open unless the action lock lever is depressed, as designed. Anyone have an idea of what's going on with the old beast? Just out of curiosity, it would be nice to know what would be required to make it normal in case of trading it off someday. Doubtful if that would ever happen, as the pump shotgun has to be about the most all around useful shootin' iron ever invented. __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.” Last edited by g.willikers; July 29, 2014 at 09:12 AM.

July 29, 2014, 09:16 AM	#6
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	G. Willikers, bet the action lock lever is worn on yours. Reshaping it with a file may help (or just replacing it with a fresh one). __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

July 29, 2014, 10:01 AM	#7
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	There must some intricacy of design that keeps the action closed momentarily for firing, and then allows the action to open to be pumped for the next round, without having to depress the locking lever. Maybe some balance of leverage, parts interaction, spring pressure or something is involved. The fact that the release operates as it should when the action is closed on a live round, but allows the action to come flying back under recoil is confusing. Unless the release is consciously activated to allow the action to open on a live unfired round, the action will absolutely not open. So far the only thing I can think of, as you say, is to just quit assessing and just change the release and spring and see what happens. Finding out how these things work enough to disassemble, reassemble and fix the obvious isn't all that difficult. However, finding the information on how they actually work in intricate detail is very difficult to come by. Maybe all will be revealed by osmosis if I just stare at it some more. __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.” Last edited by g.willikers; July 29, 2014 at 10:11 AM.

July 29, 2014, 05:25 PM	#8
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	The bolt and slide mechanism works similar to a 31 Remington on these 500's, or it is the closest I can think of. The trigger housing, and its works are slightly different. The front of the action slide lock, which has that slight taper, is supposed to be a steep enough angle that friction will hold it, even when the hammer falls, until recoil will cause the forearm/slide to pull forward slightly, and then follow the gun backwards. When it does this, it is enough to separate the action slide lock from the action slide, and allow it to drop, where the slight taper would let it release. The problem cones about if the taper gets too slick, or rounded off, and it falls with the hammer, which would allow the whole thing to start to come back under recoil, and unlock the bolt from the barrel. Winchester overcame this, on the model 12, and Mossberg similarly, by placing that 1-turn or so spiral spring on the outside of the magazine tube, which kept pressure on the action slide lock, and only allowed the recoil to slightly pull the forearm forward, while it was being held by the shooter. This allowed some wear to be taken up, and it keep working. I had a model 12 to do this very thing years ago, and the action slide lock was worn out. One might be able to straighten the angle of the taper out, on the action slide lock, if its not worn too much, and even give the face a shot with a sand blaster to increase the friction. If it's too warn, then filing on it will only make it worse, I'd imagine, as it would be shortening it, and give the action slide too much slack. Mossberg parts are generally cheap enough, that it would be much less expensive to just replace it. Also, see if the spring on the outside of the magazine tube is weak. Last edited by Dixie Gunsmithing; July 29, 2014 at 09:50 PM.

July 29, 2014, 08:35 PM	#9
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	G. Willikers - Look at that lever posted by Dixie Gunsmith. The left hand side that has that slight angle is what bears against the bolt slide. On the bolt slide, do you see where that cut out is on the right hand side. The left side of the cutout is the bearing surface that the action lock bears against. When the hammer is cocked, the action lever is pivoted up such that it contacts the bolt slide. When the hammer is dropped, the action lever is pivoted down and allows the bolt slide to move back when the forearm is pumped. __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

July 29, 2014, 09:30 PM	#10
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	Yes, it all makes sense, now. Thanks you guys for the explanation. I didn't realize that the bolt, slide and lever operated in exactly that fashion. Thanks again. With so many folks owning this model, no doubt plenty of others will learn something from this thread, too. __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.”

July 29, 2014, 10:03 PM	#11
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	g.willikers, A way to check the correct operation is by dry firing the gun. If it is correct, when the hammer falls, the action slide lock should still hold the slide. Then, if you push forward on the forearm, the action slide lock should release, (the button go upwards), and allow it to be pumped. The movement of the forearm is a very slight amount, which is pushing against the spring on the outside of the magazine tube, making the 500 operate just like the model 12. That spring is located behind a ring on the magazine tube, as is covered by the forearm, so the forearm would need removed to get to it.

July 30, 2014, 08:14 AM	#12
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	Here's an excellent video, by Midwayusa, using a cutaway model of the Winchester 12, showing the inner works and explaining how the spring around the mag tube you described works with the slide lock lever. http://www.youtube.com/watch?v=_uuMJ9grq-0 Too bad the Mossberg doesn't have it. __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.”

July 30, 2014, 08:41 AM	#13
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	That's a good video. Thank you. We had to learn about the Win M12 at school. In concept, the Mossberg action lock works the same way. With the hammer back, the slide lock is raised and hold the bolt slide back. With the hammer forward, the slide lock is lowered, allowing the bolt slide and bolt to be moved backward. __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

July 30, 2014, 11:28 AM	#14
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	The Winchester model 12 was the brainchild of Thomas C. Johnson. Some give Browning the credit for the removable barrel, but if you look at the patents, it was really a designer with the last name of Mason, I believe, that came up with the idea to use it on Winchester's lever action rifles, and was an older Winchester designer. The part Browning was involved in, was the rear portion of the 1897, and 1893, and using the Mason design for the barrel assembly. Anyhow, at the time, the model 12 was the most advanced, and better working pump designed, and I think it is until this day. During the model 12's design time, Browning had designed the Remington model 17, which later became the Ithaca 37, along with developing the Auto-5 with Winchester. Anyhow, Johnson came up with the tilting breech block design, where the action rod from the forearm worked it off a cam on the bolt, and the action slide lock came up behind the action slide rod, to block it, but used this tapered-friction holding design using recoil. The action slide, with the tilting breech block idea, was either a Browning or Remington/Pederson idea, I can't remember which, but the action slide was used in the Remington 17. Remington later used a lifting bolt lock in an no-tilt bolt, using an action slide to actuate it. In any case, I think this is about the first place I remember seeing this action slide in use, on the 17 and in the model 31. The 31 was a really great gun, as the Pederson designed 10 and 29 were bad to break down all the time. Thus, the 31 was the predecessor to the 870, and several other pump guns, like the Mossberg, and I think some JC Higgins and Hi Standards. The 17, only made in 20 ga., was sold to Ithaca, where a law suit with Pederson unfolded, and delayed its manufacturer for several years. Edit: For some of you history buffs, here are two of the William Mason patents for Winchester. It shows the use on the 93, but he had some earlier patents on the rifles, when he first came there from Colt in 1882. US499464-1 by matneyw, on Flickr W-Mason2 by matneyw, on Flickr Last edited by Dixie Gunsmithing; August 1, 2014 at 12:14 PM.

August 12, 2014, 06:58 PM	#15
4V50 Gary Staff Join Date: November 2, 1998 Location: Colorado Posts: 21,831	Thank you Mossberg I returned the old slide lock, a letter of explanation and my armorer's certificate to Mossberg. Today I received a replacement slide lock and dropped it right it. The bolt locks now and won't go back until the action released is depressed. Success! __________________ Vigilantibus et non dormientibus jura subveniunt. *Molon Labe!*

August 12, 2014, 07:29 PM	#16
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	Great! Glad it did. Earlier, when I mentioned the model 12 I had that did this, I tried fixing it by expanding the metal on the cam end of the lock, and reshaping it. I found it was better, even on the model 12, to buy a new lock. Some give warnings on replacing these, but it is not as bad as is said. You have to re-stake two small straight springs to the new model 12 lock, and simply replace it. Anyhow, I had all kinds of fit problems, including a hang fire that went off at my hip, while trying to fix the old lock, so I always say replace them if available.

September 12, 2016, 07:09 PM	#17
OKGunDealer Junior Member Join Date: September 12, 2016 Posts: 1	Not to beat a dead horse . . . I'm a big fan of Mossberg and anything American made. I just bought a model 500 in 410 and wouldn't you know? She won't lock up unless you jam the bolt closed. I checked it straight out of the box and the little lever hangs when the trigger assembly is in the receiver frame. It works fine when the trigger assembly is out of the gun. I could hear surprise in the lady's voice after she asked how l had my gun. I asked her what time it was? About 11 hours. Instantly she said she'd send a label and take care of the shipping both ways. I'm supposed to get my gun back in two weeks. My disappointment isn't that the gun was broken right out of the box. Well, maybe a little. I'm more concerned that the cause hasn't been addressed in the several years it's been happening. I'm happy that they agreed right away to make it right. That part I couldn't be happier about.

September 22, 2016, 11:24 AM	#18
g.willikers Senior Member Join Date: September 28, 2008 Posts: 10,442	No harm in beating a dead horse. It's supposed to aid in tenderizing them. This thread is only two years or so old. You should see some of them. Good info is always appreciated whenever it was discussed. __________________ Walt Kelly, alias Pogo, sez: “Don't take life so serious, son, it ain't nohow permanent.”