Mossberg 500 Light Primer Strikes

kowlguy24 · February 5, 2024, 06:07 PM

The short version is as the title states, I have a Mossberg 500c 20 gauge with the old style bolt that is giving me light primer strikes to the point of being absolutely useless (75%+ failure rate). Has anyone fixed this problem on the old style bolts (no firing pin spring)?

I acquired this gun a number of years ago used, and it worked well the first 100 rounds I put through it. However, since then things have gone downhill dramatically. I have cleaned it multiple times and tried multiple brands of shells. The hammer seems to have plenty of force to me. I basically gave up on trying to use this thing while I was in college, but now I'd like to get it going for rabbit hunting again. I called Mossberg last week, which was a pointless endeavor, and the third time that the service rep told me that there was no such thing as an old style bolt or an old style firing pin I decided it would be best to end the conversation. I would just send it in to them but I am not looking to turn my $200 used gun into a $400+ used gun if they charge me for parts, labor and shipping. Should I just send it in? Should I call again and try to talk to someone who knows more about the 500's? Has anyone fixed this problem on their own?

Thanks

bamaranger · February 5, 2024, 11:41 PM

Gosh, "failure to fire" should be fairly easy to resolve , not a gunsmith, but here goes.........

-good ammo?
-clean and lubed?
-weak hammer spring?
-damaged/shortened firing pin?

-and finally, could this not be a bolt/firing pin/spring issue at all, but instead a
headspace issue. ....... worn bolt lug, worn locking recess on the barrel extension, maybe something wonky with the barrel attachment/fitment. Ya might try a different barrel, just for grins, or find headspace gauges for 20 ga?

kowlguy24 · February 6, 2024, 06:51 AM

Thanks for the suggestions

The ammo that has failed in my gun has worked everytime in my dad's 870.
The gun is clean
In my very scientific comparison of letting hammers of different shotguns that I have acess to hit my hand, the 500 hammer seems just fine compared to the rest of them
My firing pin is 3.080" long and extends 0.07" from the face of the bolt at a maximum by my best measurement. I will try to do some research to see if this compares well to others.

Now that you mention changing barrels, I know that my buddy has a 500 20 gauge, I'll see if he will let me borrow it and I'll start switching parts until mine works, could really narrow down my issue.

bamaranger · February 15, 2024, 01:03 AM

Ammo's good and spring tension seems good.........hmmm? If possible you could measure the protrusion on the other gun and or the length of the firing pin one to another as well. Gauging headspace may tell you alot.

On the fail to fire rounds, could you describe the indentations on the primers? Does the strike imprint seem adequate or shallow? Do fired (in dad's gun) and fail to fire strikes (in your miscreant) seem similar?

Finally, I am not aware of old bolt v. new bolt styles, can you elaborate?

Best of luck with it.

kowlguy24 · February 15, 2024, 08:09 PM

I mentioned to my friend a couple times that I'd like to borrow his, still trying to see if I can get my hands on it. From what I've been able to find online the firing pin length is supposed to be 3 1/16", so mine should be fine I would think. I would say that the strikes are shallow compared to shells that actually fire.

My understanding of new vs old bolt style is that the old bolt style does not have a firing pin spring or washer while the new bolt style does have a firing pin spring and washer. The different bolt styles require different bolt slide or bolt carrier pieces, whatever they are technically called. I believe that on the new style, the pressure from the firing pin spring obviously holds back the firing pin, but also forces the bolt lock into position, or at least helps it, where on the old style bolt the bolt lock is forced into position by an interaction of the bolt lock with the bolt carrier alone in a sort of cam/rocker function. Basically when the bolt goes forward on the bolt carrier, when the action is closed, the bolt lock engages and the firing pin is free to move. When the action is opened, the bolt moves rearward on the carrier, the bolt lock is lowered and as a function of this the firing pin is pulled rearward, allowing a new round to be chambered without the firing pin sticking out.

I think that one possibility of the cause of my problem could be that if the bolt lock doesn't fully engage then the firing pin will not have its full range of motion, forcing light primer strikes. I believe that the new bolt design includes a firing pin spring to help force the bolt lock into position end eliminate this problem, but that is just my guess, I don't know if that is actually true.

bamaranger · February 16, 2024, 01:47 AM

I'm about done on this one, here's my finale. I reread your posts . It sounds as if you have actually had the bolt apart, inspected and measured the firing pin, and I would hope, super cleaned the bolt, firing pin tunnel and the trigger group. I mean like soak for a week in Hoppe's and blow it out with compressed air.

A hammer spring is an inexpensive part and easily swapped, I would do that on any older gun that was exhibiting light strikes. Deep clean and spring replacement is the logical place to start. After that one you can start investigating failure to go into full battery and headspace.

Best of luck with it.

rc · February 16, 2024, 11:38 PM

The old style bolts and carrier were for a single action bar set up. They ARE different than the newer ones designed for the twin action bars but I think you can retrofit the old ones with a carrier and bolt from a new one. They have to be replaced together. However the receive doesn't have a second hole for the twin bars so you are limited to the original pump arm. 500s have been in continuous production for more than 50 years and there have been some minor changes to the design over the years. The really old ones have a metal safety. Newer ones are plastic unless you get a military model. If you don't have the gun in full lock up because of the wrong mix and match parts, the firing pin may not move correctly.

kowlguy24 · February 17, 2024, 01:58 PM

I have twin action bars, old style bolt and carrier, and a plastic safety. Not sure what that means.

Tried to attach an image, hopefully that works.

tangolima · February 19, 2024, 11:36 AM

Haven't worked on a 500 for a while, so my memory may not be accurate.

The bolt has a locking block that locks into a square hole in the barrel extension. Not much metal to support, so excessive headspace can develope, causing light primer strikes.

Inspect the locking block and barrel extension for signs of stretching. Verify headspace by adding masking tape layers on a shell's head stamp till action doesn't close. The thickness of tape layers is an estimate of excessive head clearance.

A stretched barrel extension can be fixed by threading it on the barrel by one more turn. A stretched locking block will needs to be replaced.

-TL

Sent from my SM-N960U using Tapatalk

February 5, 2024, 06:07 PM	#1
kowlguy24 Junior Member Join Date: February 2, 2024 Posts: 4	Mossberg 500 Light Primer Strikes The short version is as the title states, I have a Mossberg 500c 20 gauge with the old style bolt that is giving me light primer strikes to the point of being absolutely useless (75%+ failure rate). Has anyone fixed this problem on the old style bolts (no firing pin spring)? I acquired this gun a number of years ago used, and it worked well the first 100 rounds I put through it. However, since then things have gone downhill dramatically. I have cleaned it multiple times and tried multiple brands of shells. The hammer seems to have plenty of force to me. I basically gave up on trying to use this thing while I was in college, but now I'd like to get it going for rabbit hunting again. I called Mossberg last week, which was a pointless endeavor, and the third time that the service rep told me that there was no such thing as an old style bolt or an old style firing pin I decided it would be best to end the conversation. I would just send it in to them but I am not looking to turn my $200 used gun into a $400+ used gun if they charge me for parts, labor and shipping. Should I just send it in? Should I call again and try to talk to someone who knows more about the 500's? Has anyone fixed this problem on their own? Thanks

February 5, 2024, 11:41 PM	#2
bamaranger Senior Member Join Date: October 9, 2009 Location: North Alabama Posts: 8,314	not a 'smith, but....... Gosh, "failure to fire" should be fairly easy to resolve , not a gunsmith, but here goes......... -good ammo? -clean and lubed? -weak hammer spring? -damaged/shortened firing pin? -and finally, could this not be a bolt/firing pin/spring issue at all, but instead a headspace issue. ....... worn bolt lug, worn locking recess on the barrel extension, maybe something wonky with the barrel attachment/fitment. Ya might try a different barrel, just for grins, or find headspace gauges for 20 ga?

February 15, 2024, 01:03 AM	#4
bamaranger Senior Member Join Date: October 9, 2009 Location: North Alabama Posts: 8,314	parts Ammo's good and spring tension seems good.........hmmm? If possible you could measure the protrusion on the other gun and or the length of the firing pin one to another as well. Gauging headspace may tell you alot. On the fail to fire rounds, could you describe the indentations on the primers? Does the strike imprint seem adequate or shallow? Do fired (in dad's gun) and fail to fire strikes (in your miscreant) seem similar? Finally, I am not aware of old bolt v. new bolt styles, can you elaborate? Best of luck with it.

February 16, 2024, 01:47 AM	#6
bamaranger Senior Member Join Date: October 9, 2009 Location: North Alabama Posts: 8,314	last time I'm about done on this one, here's my finale. I reread your posts . It sounds as if you have actually had the bolt apart, inspected and measured the firing pin, and I would hope, super cleaned the bolt, firing pin tunnel and the trigger group. I mean like soak for a week in Hoppe's and blow it out with compressed air. A hammer spring is an inexpensive part and easily swapped, I would do that on any older gun that was exhibiting light strikes. Deep clean and spring replacement is the logical place to start. After that one you can start investigating failure to go into full battery and headspace. Best of luck with it.

February 6, 2024, 06:51 AM	#3
kowlguy24 Junior Member Join Date: February 2, 2024 Posts: 4	Thanks for the suggestions The ammo that has failed in my gun has worked everytime in my dad's 870. The gun is clean In my very scientific comparison of letting hammers of different shotguns that I have acess to hit my hand, the 500 hammer seems just fine compared to the rest of them My firing pin is 3.080" long and extends 0.07" from the face of the bolt at a maximum by my best measurement. I will try to do some research to see if this compares well to others. Now that you mention changing barrels, I know that my buddy has a 500 20 gauge, I'll see if he will let me borrow it and I'll start switching parts until mine works, could really narrow down my issue.

February 15, 2024, 08:09 PM	#5
kowlguy24 Junior Member Join Date: February 2, 2024 Posts: 4	I mentioned to my friend a couple times that I'd like to borrow his, still trying to see if I can get my hands on it. From what I've been able to find online the firing pin length is supposed to be 3 1/16", so mine should be fine I would think. I would say that the strikes are shallow compared to shells that actually fire. My understanding of new vs old bolt style is that the old bolt style does not have a firing pin spring or washer while the new bolt style does have a firing pin spring and washer. The different bolt styles require different bolt slide or bolt carrier pieces, whatever they are technically called. I believe that on the new style, the pressure from the firing pin spring obviously holds back the firing pin, but also forces the bolt lock into position, or at least helps it, where on the old style bolt the bolt lock is forced into position by an interaction of the bolt lock with the bolt carrier alone in a sort of cam/rocker function. Basically when the bolt goes forward on the bolt carrier, when the action is closed, the bolt lock engages and the firing pin is free to move. When the action is opened, the bolt moves rearward on the carrier, the bolt lock is lowered and as a function of this the firing pin is pulled rearward, allowing a new round to be chambered without the firing pin sticking out. I think that one possibility of the cause of my problem could be that if the bolt lock doesn't fully engage then the firing pin will not have its full range of motion, forcing light primer strikes. I believe that the new bolt design includes a firing pin spring to help force the bolt lock into position end eliminate this problem, but that is just my guess, I don't know if that is actually true.

February 16, 2024, 11:38 PM	#7
rc Senior Member Join Date: April 28, 2001 Location: CA Posts: 1,769	The old style bolts and carrier were for a single action bar set up. They ARE different than the newer ones designed for the twin action bars but I think you can retrofit the old ones with a carrier and bolt from a new one. They have to be replaced together. However the receive doesn't have a second hole for the twin bars so you are limited to the original pump arm. 500s have been in continuous production for more than 50 years and there have been some minor changes to the design over the years. The really old ones have a metal safety. Newer ones are plastic unless you get a military model. If you don't have the gun in full lock up because of the wrong mix and match parts, the firing pin may not move correctly.

February 19, 2024, 11:36 AM	#9
tangolima Senior Member Join Date: September 28, 2013 Posts: 3,829	Haven't worked on a 500 for a while, so my memory may not be accurate. The bolt has a locking block that locks into a square hole in the barrel extension. Not much metal to support, so excessive headspace can develope, causing light primer strikes. Inspect the locking block and barrel extension for signs of stretching. Verify headspace by adding masking tape layers on a shell's head stamp till action doesn't close. The thickness of tape layers is an estimate of excessive head clearance. A stretched barrel extension can be fixed by threading it on the barrel by one more turn. A stretched locking block will needs to be replaced. -TL Sent from my SM-N960U using Tapatalk