[spaghetti]

My Excalibur Carbon has external diameter 28mm and a wall thickness of 1,5mm. I assume that the hole is 26,5mm.
I'm just guessing, but I guess that the HH size A (smaller plug) is intended for barrels with thicker walls (Aimrite, Rabitech) and the B size (fatter plug) is for skinnier barrels like the Omer's.
I'm just guessing, but this makes sense to logics...
If it was for me, I'd get the fat type: you can always use sandpaper to scracth off some material if too big, while you can't add material to make the slim one fatter...epoxy is just a glue, not solid stuff...
However: try to send a message to Mattedhead through Deeperblue or Spearboard. He's a great guy and no doubt he'll help.