N140 has shot very well for me under the 77 SMK. Even with blems, I was getting well under MOA out of the bolt gun with a 223 chamber.

77 SMK, 24.0 N140, LC brass, Federal 205, 2.270 OAL.

I would move to a CCI450 if I bumped up the charge. With a 24" barrel, there isn't a need to push them hard.