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As the barrel heats up it can cause the bullets to scatter on a target. Some people time their shots at the range allowing 3 to 5 minutes minimum between shots when testing to choose a load.
HAPPY HUNTING
Point of impact differences at 100y is as a result of barrel vibrations and not bullet drag. If your barrel is touching the stock as in 'not free' floated that could cause stringing. Glass bedding and free floating could help consistency. Did you give the barrel time to cool? If you did not give the barrel time to cool take your time and shoot a 'very' slow 3 shot group and see if it make a difference. I would recommend a good rear bag also.
Hot barrels don’t “walk” shots or “scatter” shots unless there is something wrong with the barrel. A properly stress relieved barrel will continue placing rounds within the true group size until it melts (until it gets so hot that you have barrel droop).
While I agree that barrel heat doesn't necessarily cause shot scattering, it can cause stock material to heat up expand and push the POI one direction or another. My first bolt rifle was a 7mm RM Ruger and when I tried to sight it in the POI kept climbing up and to the left. Removed the 'speed bump' at the end of the stock to free-float the barrel and now it maintains a consistent POI. That was back in 1982 and I've been free-floating my wood-stocked barrels ever since.
The downside to free floating is that group sizes for the first few shots from a cold bore may increase slightly. I consider this an acceptable trade-off fr a barrel tht doesn't change POI due to temperature, humidity or other factors.
That said, I'm not sure that the problem experience by the OP is due to a non-free-floated barrel. Do the easy things before the harder things.
1. I think navlav8r said it first - make sure nether the pistol grip of the sling stud hit the bags during recoil.
2. Check the table for stability. Get a different table if necessary.
3. Make sure the scope and mounts are tight.
4. Make sure the action is properly torqued to the stock.
5. Start with an absolutely cold bore and stock. Shoot 3-4 shots and see if the results are the same. Do it fairly quickly to minimize stock expansion due to barrel heat.
6. Try different ammo.
7. Have someone else shoot it and see if the results are the same.
8. Free-float the barrel and reseal the stock with polyurethane. (This is typically a 20 minute job.)
9. Try a different scope.
10. Bed the action.
11. Change the barrel.
