i think that "too short" and the increased chamber pressure comes from compressed loads - check your depth from case neck down to the powder level, and determine how far you can seat your projectile before you have a compressed load. Anytime you seat deeper than that, successively, you're compressing more, and increasing chamber pressure more.
Also, seat a projectile in an empty case - and seat it long - much longer than magazine length. Don't crimp the case, at all. Fully chamber that round by hand. This will tell you how long you can seat it, and just hit the lands. Seat shorter than that.
Your ultimate restriction is magazine length.