I had my first run in with this about 1973. My neighbor brought me his grandsons 243 Win.. Said it shot good. The grrandson had someone do a glass bedding job on it and it shot bad now. I took it out of the stock. (Getting the action screws loose is another story!!!!!.) I guess from the looks of the screws they never got it apart to check the bedding. The action was bedded good. But the barrel had air pockets here and there, 1 air pocket on one side of the stock right at the fore end. I cleaned out the barrel channel, Free Floated the barrel,
resealed the barrel channel and left the action glass bedded.
Loaded 85 gr Sierra bullets, DuPont Powder and CCI LR standard primers, All I had on the shelf.. It shot 1 1/4 to 1 1/2 groups. Same box of bullets, Same can of powder. Got some Rem LR primers. When I returned the gun, The grandson asked how it shot. I was ready for this. Laid the 4 targets on the table. Pulled out 4 dimes and covered all 4 groups. HE was happy.
1994, My Brother and I was getting ready to go to Canada on our first Moose hunt. He was talking about his 338 WM, 225 gr. bullets shooting a 5 shot group at 100 yds.. 3 or 4 in 3/4" 1 or 2 Low Right about 4 inches and him blaming himself. We was at a local range one day. and a guy had just bought a little Chrony Chronograph, Asked if we would like to shoot through it. I said brother here is your chance. He shot 5 rounds through it. 4 shots good group, The 3rd or 4th shot went Low Right about 4 inches. When we looked at the chronograph 4 shots ran about 2960 to 2975, The shot that flew was 2795. The primers he was using was CCI LR 250. He bought CCI LR 250 from another Lot and they shot good groups without the flyer.
I have a load for my 223 Rem. that shoots 1/4 to 1/2 groups. Hornady 50 gr. V-max, IMR 4198 and Rem 6 1/2 B primers. Real close vel. shot to shot. Tried Win. primers. Group opened to 1". On the hunt of more Rem. B Primers.
When I go to the range, My chronograph is an important tool to take and use.
It tells you what is going on, Cuts down the head scrathing and wondering too.