I have assembled several SBR's after an approved form 1 is in hand. The caliber on the receiver matched the caliber stated on the ATF form 1, and consequently the designated ammunition was utilized in that SBR. Today I submitted a form 1 on the ATF E-Form site. The only caliber displayed in the pull down menu was .223. So that is what well appear on the approved form 1. Talking to the Class 3 FFL I transfer thru, he said so long as the barrel has "9MM" engraved on it. Everything is good.

Jeff, well you please make sense of this hole thing. What's stopping me from modifying my SPIKE's Tactical SBR from .223 to 9mm and back again ?