Primers are the hardest thing sometimes to figure in loading/ reloading shells. Sometimes you get primers that have no rhyme or reason to them. If the priming solution the manufacture is putting in the primer is to thick to thin or something causes the primers to not be filled with the exact same amount of priming material.
About 1993 my brother and I was working up loads for our 338 WM's for our first moose hunt. He was having a problem every 8 or 10 rounds he would have a shot go 6" out of his group. He was blaming himself. We was at a local range one day and one of the club members had just got a Chronograph. He asked if we wanted to shoot through the chronograph. I insisted my brother shoot his rifle. With 225 bullets, IMR 4350 the first 3 shots ran at 2970-80 fps. went in 1" at 110 yds. His 4 shot 6" low and to the right. Looked at the chronograph the vel. was 2760's fps.
I gave him a 100 flat of primers and no more problem, He threw the rest of the primers over the hill.
I have a load right now my Rem. 700, V. 223 Rem. it will shoot .250 @100yd. groups with a load I developed. The load uses Remington small rifle primers. I tried Winchester and CCI primers in the load workup, No good results. Now with the fall of Remington, When I run out of Remington primers the rifle may only be a good Tent Stake.