Hi Dwayne,
Eileen and I have a fair amount of experience with the 6.5x55 and .270 with X-Bullets, of various sorts from the "plain old original X's" onward, as well as other bullets.
I would guess the difference you're seeing is indeed due to the rifling twist, which definitely affects bullet expansion. As I recall, the Swedish rifles have a fairly fast twist, faster than the 1-9 found in many American rifles.
But even the 1-9 in the Ruger 77 Mark II 6.5x55 I used for several years really made some impressive holes and quick kills on deer and antelope when using the 120 X-Bullet at 2950 or so. Probably the kill that surprised me the most was a pronghorn at almost 400 yards. With a pure broadside long shot, no bone other than ribs involved, he went down within a few feet. Can't remember any animal hit with that load going more than maybe 35 yards, while several shot with the .270 and the 140 TSX at 3000 fps went more than 35 yards, and one pig went close to 100 yards.
As always, however, shooting stuff isn't an exact science!
Good hunting,
