What length firing pin do I need?

[tinlizzy]

Have an old (not sure exactly how old - but at least 35+ years.) 870 12GA shoots 3" mag shells, sn/ S222836M. Firing pin is broken, lost a piece in the field and don't now if I need 2 15/16" or 3 1/16" Any input on how to tell? Not a gun expert. Thanks...

[bigblue]

2 15/16 . 3inch chamber has no bearing on pin length.

[tinlizzy]

Thanks bigblue... Appreciate the help.