AAC threading guide -- give a set of these to your gunsmith.

General Firearms NFA
1

When you print these, best to set your laser printer on 1200dpi. You can do that in the advanced printer preferences.

guide 1

guide 2

guide 3

guide 4

2

Robert,
What thread pattern will the first Blackboxes have when they ship? Do you know of any barrels that are available in this pattern for a G21?
Thanks:D

3

It will be the same patterns as the Evo-45.