Again, the OP says problem occurs where the cartridge is (not) stripped cleanly/completely from the magazine to enter the chamber. If the round chambers freely after that (i.e., the bolt closes), it is not a small-base sizing problem.
QL tells me 27gr BLC2 under a Sierra 55gr FMJBT w/ an OAL of 2.230" produces a shade over 50,400psi, which is not an overly-aggressive load. (Hornady max's out at 28.1gr BLC2 -- which may be also be case capacity limit.)
My bet it still on a magazine-related phenomenon. (By any chance do you use the magazine for a left-hand hold point?)