The very best of the bolt carrier groups. The highest may be the conventional five.56mm bolt provider group. Notice the bolt, cam pin as well as carrier crucial. On the bottom is the 9mm SMG bolt group. I have not decided on a flat top or A1 (with no FA https://coltsmgupperreceiver08528.shotblogs.com/new-step-by-step-map-for-colt-smg-upper-receiver-40643464