Unless Brian's got a new toy and is giving it a test run...he usually has lots of BBs on hand.
Yes, TTAC3 is 0.20 only. It's tight/close enough that anything else is unnecessarily painful. The entire length of the area is just enough for the hopup to kick in on most rifles...so heavier weights for accuracy/distance doesn't really matter.
A bag of 0.20's will go a long way there. He's usually got some other weights (0.25/0.30's for sure...don't know about the others).
(note: I think he might have a bunch of bags of "junk" bbs (new discount ones that were a bad batch..not BBbastards) as well...perfect for grenades)