Fairly new build with a 10.5 KAK melonite barrel.  Free floated with a MI SS2 tube and a screw lock KAK low profile gas block (not touching or rubbing).  Also using a small KAK Flash Can

Runs great with no problems shooting 55 gr Wolf or 62 gr PMC green tip.

Only issue I have is that POI shifts vertically about 2-3 inches at 50-ish yards when it warms up.  No real change from warm to HOT after some fast shooting.  Doesn't seem to move L/R at all.  

Anybody else see this?  I would have thought the shorter barrel would be less susceptible to movement.

Is this with irons or an optic?

Irons right now.  

Daniel Defense fixed front and rear.  Both on the rail.  Not on the GB or barrel.

I'm wondering if it could be the rail itself heating up and moving? That or possibly an issue with the barrel nut or front of the receiver where it contacts the barrel extension. Honestly the rail gets my vote I doubt it's the barrel.

Interesting idea.  Did not even think of that.  The MI tube does have a full length rail to it so it's thicker on top.  Might be expanding a touch more on the bottom than the top.  Or the other way round to push POA lower.