benchrst

1: You got a crap box of primers (unlikely) 2: Your striker channel is packed with crap / weak spring / weak hammer spring (more likely) 3: Primer not seated deep enough (most likely) .02

A bit apples to oranges, but might be helpful. 10MM Ba 7.5 / 9.10gr / 155 MG / 1.255 / new Starline / CCI 300 / 54 degrees / 1,156 fps N350 / 9.00gr / 155 MG / 1.255 / 1X Starline / CCI 300 / 54 degrees / 1,218 fps

If your intent is to reload the Armscor brass I would do some research, I've read some sketchy things. If the brass is bulged, then it's compromised, especially if there's a smile line (shear). That said, mild loads shouldn't be a problem. As recommended, try whatever sizing die your end up buying, and if they don't plunk get a bulge buster (needs the corresponding FCD),

The following are guesses, and might not be very good ones Your OAL has the bullet very near / touching the rifling. The bullet OAL varies enough to effect pressure (seems pretty unlikely). Bullet has a residue that's effecting grip.