Here we go....
Properly built rifles do not have “poi shifting” no matter how thin a barrel is, and no matter hot it gets. That is a myth. Uless the rifle needs work (not properly stress relieved barrel, bad bedding) when you see groups “shift”, get larger, etc., what you are seeing is the true grouping capability of the gun. Nothing more.
5 shots isn’t a group either. Grouping is for a probability matrix. Rifles shoot in a cone, they do not shoot in a “hole”. That cone’s size tells one what size target the gun is mechanically capable of hitting, and whether the cone is centered on point of aim (zeroed). That’s it. 3 shot groups were invented to make people feel better- literally. Writers started doing 5 shot groups instead of 10 round groups decades ago when reviewing rifles to make them look better. Same from 5 round groups to 3 round groups. It has nothing to do with reality.
Take your “.65 MOA” rifle, lock it in a vice in an indoor tunnel and fire 100 rounds. It will not be a .65 MOA rifle. If it “averages” .65 MOA, then some rounds are far worse. Those are the rounds you want to know about if hitting matters. I do not care where my best rounds go, I need to know where the worst ones will go.
Statistically your .65 MOA average rifle can only on demand hit a 1.5-1.7 MOA target.
This matters for several reasons. One- 10 round groups show the true cone, and consequently the target size that the rifle will mechanically hit. Two- it shows if there is a mechanical problem with the rifle that needs addressed. Three- it allows a true zero. Fourth- when newer shooters hear “sub MOA all day long” they are being lied to, and can be confused like the OP was.