Riddle me this- upper fires on one lower but not another.

[Bergie]

Went to the range yesterday and to save range bags/weight threw in a lower and two different uppers. (5.56) One lower and upper worked as expected, but on the other lower, the firing pin just created a small dent in the primer. Yet I pulled that lower off a lower I used with it back in Nov and if worked just fine. Why would that be? The hammer striking one pin hard, then soft on another pin?

[crbutler]

Too much lube/clp on the firing pin- gels when cold?

That cost me a nice deer this year.

[Bergie]

Interesting, never considered that. Haven’t torn down the bcg but will today and wipe her all down….will load a dummy round to see if it make a good dent

[Rip Van Winkle]

Do the uppers have the same brand/style bolt carriers? Were they both clean?

Do the lowers have the same hammer/trigger assemblies?