I had this problem when I got my gun back from the firing pin refit. It would not detonate milsurp like XM193 and M855 100%. Maybe one round out of 30 or so would not detonate.
I removed the captive spring from around the firing pin and have since ran about 2000 rounds of various milsurp through the FS2000 without any failures to detonate other than plain dud rounds.
In attempting to cure the FS2000 of slamfiring commercial primers, FN went with both a captive spring and a lighter firing pin. This was overkill and resulted in a gun that would not reliably detonate some milsurp. By simply pulling the captive spring off the firing pin, you will have a firearm that works 100% with all milsurp rounds but will not slamfire.
I removed the captive spring from around the firing pin and have since ran about 2000 rounds of various milsurp through the FS2000 without any failures to detonate other than plain dud rounds.
In attempting to cure the FS2000 of slamfiring commercial primers, FN went with both a captive spring and a lighter firing pin. This was overkill and resulted in a gun that would not reliably detonate some milsurp. By simply pulling the captive spring off the firing pin, you will have a firearm that works 100% with all milsurp rounds but will not slamfire.