A couple minutes between each shot on a magnum is not bad. You will
have a different group with a warm barrel as opposed to a cold barrel.
A cold barrel is the temp of the barrel acclimated to its environment
before you even begin shooting. Your cold barrel temp is different in
July from that of Decemeber.
A few things happened here:
You didn't get back down to a truly cold barrel. Had you waited
for that to happen between each shot you would have found a
grouping of rounds in a similar spot (all else being equal) to your first.
However, as your barrel warmed up your point of impact changed.
When you hit a consistent "warm" point you then began to group in
that spot which was, of course, different from your cold barrel poi.
Another big thing here is the practice effect. You were probably shooting
better after a while as you got use to it. A 300mag has some recoil.
If you plan on shooting frequently, then stick with your warm barrel
zero. If you plan on hunting where you will get one shot at a long
distance, you will need to learn your cold barrel zeros.