I'm going to have to pick up some TAC. I keep meaning to give it and X-terminator a try. Besides, the other twenty-odd powders I have are looking lonely.
One season I shot AA2520 in my M1A. I found the irregularity problem with it, too. It proved to be due to ignition. I was using 210M primers at a time, and found that if I deburred the flash holes, the inconsistency was reduced and groups cut from 1.25 moa at 100 yards to 0.70 moa shooting from slinged-up prone. Significant. You might give that a try with BL-C(2)?
How big are the Tac grains compared to BL-C(2)? My flash hole deburring never made any difference with stick powders (which all worked pretty well in that gun), and I think the larger spaces between the grains just let the flame get around so much more easily that it is less particular. I wondered if Tac had larger grains that made flame propagation easier? Of course, 748 is very fine grained, and it doesn't seem to be unacceptably picky about ignition.
Incidentally, back when I did the 2520 trial, I wasn't aware that what magnum primers do is raise start pressure for hard-to-light powders. I'd thought it was just flame burn time or quantity. It might be it would have done the same for me as deburring the flash holes did? It's something elese to try when you have trouble with irregular ignition.