The mixed head stamp brass will definately give you a little more variance. I am loading 9mm for minor power factor to 1.14 oal. I give myself an allowance of +/- .005, so 1.135-1.145 goes to a match, anything outside of that within reason goes in practice ammo. On my 550 I get an occasional round outside of my allowance, but it’s not the norm. If you are getting a large quantity way outside of spec., I would be investigating further, otherwise I would chalk it up to the mixed brass.