It's usually shooter error but sometimes can be ammo related.
5-6 years ago at a gun show a guy was selling some off name .38 158 FMJ stuff. IIRC it was $5/50 rds. Not reloads. All the writing on the box was Portugese or some such but it came out of Brazil.
Fired the first 6 rds out of a 2.5" Model 19 at 25 yds. The first thing I noticed was they were hot. Checked the target and 4 were centered about a 2" group. 2 other rounds were a foot away from the group of 4. Those 2 rounds were keyholed. Fired 6 more rounds and exact same results. 2 keyholed away from the group. Loaded up 6 in a 6" Model 28. 3 in a group and 3 a foot off and keyholed.
Still noticed they were hot. I loaded 3 of the .38 FMJ rounds and 3 .357 mag rounds. I couldn't tell a difference between them. Same amount of recoil. I pulled the bullets on the remainder of the box and weighed them. They all appeared OK but weights were off by up to 5 grains each. That small difference shouldn't have caused the keyholing tho unless the weight was distributed to one side. Pitched them.
Later on thinking about I wished I had mic-ed them to see if they were undersized. I have loaded some 85 gr bullets for a .380 in .357 rounds just for play and never had a bit of problem with them grouping or keyholing.

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183rd FBINA