He did mention clean barrel, and I realize fouling affects velocity. But I'm saying that I've seen this plenty in barrels which are fouled already.
A friend I work with is always bringing in ragged hole 3 shot groups from his stock Sendero 7RUM. We're talking .3MOA and below. He's pretty sure I'm pure stupid for spending 5 times as much for custom rifles
. He does very basic reloading and shoots off a carpeted 2x4 and sock/sand. Says he can do it all day long. I always wondered how because I couldn't make such a claim off a Farley with perfect 6br ammo.
One day he invites me to his range, and I find that he is not pulling my leg. He can do it. He fires 3 shots in about 15 seconds. Then, lets the barrel cool(1hr) for the next group. He worked up his load this way, and I damn sure wouldn't want to be a dear in his crosshair.
Anyway, I mentioned that BR shooters put down 5 shot groups, and challenged him to try it. When he finally did, sure enough, the last couple moved away from the pack. After another damn hour, I had him shoot 5, with one minute between each. With this he shot 3/4". I told him to move to another target and keep shooting at 1min intervals now that the barrel was warmed up. Doing this his groups reduced some to ~1/2". If his load was worked up for hot barrel shooting, it would probably do better.
He didn't do anything wrong, and didn't need to change anything. It's a excellent hunting rifle, and not a benchrest rifle. Thats what he learned, and I learned alot as well.
If you have a barrel that doesn't shoot differently from cold to hot, it's probably cut rifled, and lapped AFTER contouring. Or it's damn good for some reason unknown to me.