Can you change calibers on a SBR
I have a 6.8 spc upper and have received my approved forms for both a suppressor and SBR. My NFA dealer said the threads on my PRI upper are not compatible with the 6.8 AAC suppressor threading. I advised that if he was going to rethread it - go ahead and cut it to 10.5". He hesitated and said since my form 1 said the lower was a .223 (Colt) he would need clarification from the ATF if you can put a 6.8 on a lower marked .223. I thought the lower is what is registered -not the upper.
Can any of you help me with this question?
In advance, thank you.