If my lower says “multi” but my form 1 stamp says 5.56, can I put a 300 upper on it ? Or am I stuck with 5.56 upper only ? Should I have entered multi on the form 1 ? Or should I just make another SBR slower to be safe.

They won’t accept “multi” any more, even if it says it on the lower. You have to specify a caliber. After it’s approved, you can change to uppers of different calibers. It’s recommended that you retain an upper with the registered caliber.

Dwill104 is as usual correct. If you don't want to retain the upper that put the SBR in the configuration that it was built you can notify the ATF of a new configuration that you do retain.

cool thanks guys. It's registered as 11.5 inch. I have several 11.5 uppers as well as 10.5 and 12.5. all 5.56 but not planning to get rid of any. just concerned about the issue of my lower saying "multi" and my file tax stamp saying 5.56. but if it not changeable then so be it.

I found a 9 inch 300 blackout including BCG for $200. I just want to stay perfectly legal.

the only things that matter on your lower at this point are your name, city and state and the serial number. everything else at this point is completely meaningless.