E-Form 1 instructions specifically stated that the ATF wants the base caliber, not specific variation.
They even used 300 BLK -> 300 as an example. You probably picked 300 from the pull down list (IIRC it was a pull down list). You are fine.
From the 9/17/2013 eForms Bulletin (Page 3):
Quote:
Calibers in NFA
NFA uses only the ‘base’ caliber
for describing an NFA firearm
and no ‘.’ to start. For example,
we use ‘22’ rather than ‘.22,’
’22LR,’ .22 WIN,’ etc. Similarly,
we use ‘300’ rather than ‘.300,’
‘.300 Blackout,’ ‘.300 AAC
Blackout,’ etc.
For metric descriptions, we do
include the ‘.’ when it is internal
to the ‘base’ caliber, such as
‘5.56’ or ‘7.62,’ but we do not
use the further description to
the caliber such as ‘5.56x45’ or
‘7.62x39’ for example.
Also, how would a form 1 get kicked back for listing 300 as the caliber? The ATF doesn't know what the person is building.