ORIGINAL: statjunk

I've taken this Ruger Mark II in 300 WM to the range now for about 7-8 weeks in a row. Here is the bottom line of what is happening:

The first 2-3 shots are less then a half inch apart on average. The rest of the groups that I shoot for the day have relatively very poor accuracy. It really doesn't matter all that much which bullet combo I shoot with the IMR 4831 it shoots the first 2-3 really well. I was also getting the same effect with RL-22 loaded really hot, 76.5gr. I've let the rifle cool at the range for over a half hour and still the groups suffer. However by the next day the gun shoots again.

Does this indicate that the heat treatment on my barrel is flawed? Do I need a new barrel?

Some additional info, rifle is bedded, floated and has a really great trigger.

Tom