This was pulled from another thread that RSILVERS commented on the thread was about the correct wording for this round.
Ideally it would say:
Also don't put zombie hunting in the reasons on another forum a guy put zombie hunting in for his reason and it delayed his form until he fixed it.
Sure. Mine says 5.56mm. Mainly because it started out as an SBR 10.5" 5.56 but since you don't have to do any paperwork changes when switching calibers or barrel length, it doesn't matter.
With one caveat... you must be able to return the rifle to it's registered condition. ie. I have to maintain the 10.5" 5.56 upper in my inventory.
As far as your situation, I'd put 300BLK and call it a day. Caliber isn't a major hanging point from what I know of the process (4 SBR's and counting) Just make sure all the boxes are filled in and you don't put "Whacking Tango's" in the "Reason for application" box.