Regarding point of impact changing as barrel heats up, the most common cause is not the barrel. It's the receiver face not being squared up to the barrel tenon axis. As the barrel heats up and expands, it bears harder on one place around the receiver face. That causes the barrel to whip more in that direction.
When all is squared up correctly, no point of impact change across 10 shots fired once every 5 seconds in highpower rifle rapid fire matches. In one test starting with a cold 308 Win barrel, 40 shots were fired about 10 to 15 seconds apart and the extreme spread at 600 yards was 1.9 inches
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