Mr. Wallace QUESTION: My rifle has a nasty habit of shooting high for the first cold bore shot of the day. The 4 follow up shots will be in around .300. You can imagine how frustrating this is. Have you ran across this with any of your rifles ?
Rex: Yes, sir. That is quite common. There are a few possibilities:
1) Clean-bore deflection - This effect is commonly associated with bores that have just been cleaned... it usually takes a few shots to reestablish stable bore conditions after a bore cleaning as the fouling builds up to equilibrium point again.
2) Action shift - Another possibility is that, if a rifle has just been removed from a gun-rack or your case and is then carried to the firing position, it is possible that the action and recoil lug have shifted (ever-so-slightly) forward in the stock during transport - after the first shot, the recoil lug slides back again and again become solidly mated for the follow-up shots. This effect could be mostly eliminated from the equation by a glass bedding job.
3) Muzzle Velocity Variation - Sometimes the first shot on a very cold barrel can warm up the chamber to a more standard operating temperature for the day and when you feed the rifle after that the ammo is brought to a more consistent warmer temperature effecting MV variation.
4) Barrel Warp - Temperature can also play a role in POI shift via 'heat warp' in a barrel in which the bore hole is extremely off-center, but its usually a very small shift and wouldn't occur until the barrel heats up significantly to induce warp