Mossberg 500 mis fires

[Dand]

I'm seeking advice:
I have a 12 ga Mossberg 500 (1991 ) that doesn't always hit the primer hard enough to fire. I notice this the most on the trap range. For a while I blamed the shells from an old supply.  But they always fire in the 870.  But recently I had one non fire in some fresh reloaded shells. Only once in 2 rounds of trap.

So I took the bolt apart and cleaned thoroughly.  Really scrubbed it.  No gunk build up in the firing pin area.  Seemed to help a little but still some mis fires.
So I took apart the trigger group and again scrubbed thoroughly and reassembled.  Still a mis fire or two. I the hammer drop too soft?

I ordered 2 main springs (drives hammer)  from Gun Parts - one was supposed to be OEM made and one a "reproduction" version. Both supposed to work in a Moss 500.

one came and is too short, shorter than the original by maybe a 1/4 inch.
The other one is too long by maybe a half inch. I was reluctant fool around and cut coils.
So I tried stretching the old spring just a little and reinstalled it.

I even bought a whole new bolt as the original one, a pin stuck and I can't remove it.

I haven't test fired it yet but have to wonder if a tiny bit of stretch would help much or for very long.

I'm open to suggestions of what to do or what might be my issue.

[Graybeard]

Sounds like you've covered most of the things I'd have recommended. I don't see that you've replaced the firing pin. Since that is what is responsible for hitting primer that's where I'd go next. I'd also be tempted to use that longer spring you said you have if firing pin isn't the fix.

[Dand]

Thanks Bill. I may have to try a new firing pin.  The longer spring is much too long I fear and likely would be a total jam fit. I tried installing it but no way. I think I'd have to take off a coil or 3.

Thanks.  I'll send off for a pin next.

[Dee]

I've had shotguns (especially trap guns) brought in where the firing pin had peened down over the years. Thats where I'd look.

Off topic, but I carry Smith revolvers daily. I have installed extra long firing pins in both.

[jedman]

About all I can think of would be to check the firing pin tip protrusion at the bolt face. Should be about the thickness of a penny layed next to it with the back of the pin flush with the back of the bolt. Otherwise you may have a excess headspace problem where the shell can move forward slightly when struck by the firing pin which would mean the rim recess is cut slightly to deep.
jedman

[ironglow]

  My grandson often used a Mossy 500 when clearing terrorist "safe houses" in Iraq.  I thank God the Mossy was ultra dependable for him...   

  It also sounds to me like it's the springs, try to get originals.