RE: Question about barrel length in 7 mag.
Sounds like your rifle doesn't like that bullet, or that weight of bullet. The bullet might be heavier than what your rifle likes. Try a 140 grain bullet and/or try a different bullet. With a barrel as short as yours the velocity is probably in the range of a 7mm-08 or a touch faster, so you may consider a 140 grain Nosler Ballistic Tip. They are well renouned for their accuracy, and should work well at the velocities your gun likely produces.
Good luck.
Mike