Hmmm, I had assumed that it followed the relatively common convention that the clubs designated Battle Group were Big Gun (like the MBG, NTXBG, CTXBG, SCBG, CBG, ESBG, AUSBG, and MABG), while Task Force, Attack Squadron, etc. were Small/Fast Gun.
Sorry for the confusion.
However.... NABGO is, indeed, an Open event, and Small/Fast Gun ships are welcome/invited to battle under the speed and ROF rules in place. We should have a Small/Fast Gun HMS Invincible making her NABGO debut this year, in fact. She did sea trials at the NTXBG Father's Day event in Weatherford.
Cheers,
Wreno