Working up loads for my 1:14 38-55 with .376 bore diameter.  Until this weekend I was getting mediocre results with a 312 RCBS bullet.  Upon suggestion of more experienced shooters I sized them to .378 (to ensure concentricity more than anything) and switched to pistol primers, rather than magnum rifle primers.  Groups shrunk significantly and were very consistent.  Shooting off a rest groups went from scattered holes (no key-holing though) on a NRA 200 yard paper target to very consistent hits on a 10" steel plate at 200 yards (open sights).  Would changing primers have made that much of a difference, or sizing the bullets?  RCBS indicates this mould drops at .377-.378.  They were dropping at .379 for me with WW.  I know changing only one variable is the accepted protocol, but I'm trying to speed the process up. I will continue to refine this load using 45 grs Goex 2F and bullet seated out to the lands.

Yes to Pistol primers. I have a couple of 45/70's that like pistol
Primers so well that I swaged the brass in the primer pocket so they only take pistol primers. Accuracy went up and no slamming against the breech block. All is good.

Have you tried softer alloy? I use pure lead or 30-1 with my BP reloads and BP lube.