This may sound stupid to those of you with far more experience than I shooting larger bores, but my question is this. I have observed that when I take my first couple shots with my .375 H&H that they go exactly where I'm aiming, but that after a few rounds they hit low, a pretty consistent 2" at 100 yds. The only difference I can see is that the barrel has heated up after those first few shots. What could cause this?

I've been shooting for several decades, but never anything larger than .30 cal catridges before this, and I don't recall ever experiencing this phenomenon.

My guess would be that you have contact between your barrel and stock. As the barrel heats and comes in contact with the stock it is affecting point of impact. You should be able to slide a piece of paper between the barrel and stock from the end of the stock to the reciever when the barrel is cold or warm, if not, get a kit and free float the barrel and bed the action. Do a web search for kits and instructions.
Hope this helps.

Redmule,

Hadn't thought of that. Thanks for the suggestion!

Is your bore always freshly cleaned for these first 2 shots?

ledvm,

No. I thought of that too. I've fired around 60 rounds or so through the barrel, but not always under the same circumstances, and have not cleaned the bore yet. It definitely seems to be related to the barrel heating up.

Cheesehead ,

Your .375 may have a secondary lug on the barrel which will not let you slide a piece of paper the length of the barrel. I suspect the barrel comes in contact with the stock as it heats up or the secondary lug needs to be bedded.

Buliwyf,

I checked the rifle this morning and a piece of paper does slide from the front end of the forestock to a spot just under the back sight, and then from just to the rear of that to the end of the receiver. Does that sound like what you were thinking of?
In a hunting situation I would think none of this should be a problem, unless I empty out on something big and pissed and then need to worry about quickly following up with accurate shots. Of course, that's one of the reasons PH's carry big guns themselves.
I haven't tried carying it around in the hot sun for hours and then shooting it. Wonder what that would do?

Cheesehead and proud of it!!