From what I've read, the ATF doesn't like nonsense on the Form. Multi caliber, ranges of overall lengths, multiple barrel lengths, etc.

Pick a caliber and pick an OAL and Barrel length. Once you are approved, the gun IS AN SBR. You can make temporary barrel and caliber changes all you want.

If you want to prolong your application process add a bunch of nonsense on the specifics... DIG?