Benbo,

Yeah, looks like your velocities are pretty much normal for the unimproved 6mm! In fact I've gotten similar velocities with a couple of .243 Winchesters that had 24" and 26" barrels.

Have never fooled with the 6mm AI, but have with the .257 Roberts AI, which is a very similar case. The improved version had about 8.5% more powder room.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck