shot my new sig mk-25 ( P-226 ) today with what I thought would be some good loads. Using Lyman 356402 tire wts. PB with magma lube and sized .356. Weighed 122-123 grs. Loaded with cci-500, and 3.9 grs, 4.0 grs and 4.1 grs of bullseye powder. 20 rds each with each powder wt. 10 of each were taper crimped with Lee factory crimp die and 10 of each were not crimped. Used virgin starline cases for all. Several of these loads keyholed in target.
I've previously shot same bullet with 3.8 and 3.5 grs of bullseye with perfectly round holes in tgt. at same distance. Have also use Acc #5, 231,and mostly power pistol powder. The bullets on the last listed powders was linotype and weighed 111-113 grs. and all holes in targets were perfectly round. I've cleaned bbl after each shooting session so no copper or lead in bbl when I shot today.
Any ideas on why I experienced keyholing today would be appreciated. Thanks.