My Steyr will walk up from heat almost 1 MOA if I shoot it hot. Hot is a relative term, if I shoot a quick three shot group, the last shot of the group will invariably be a little higher than the other two shots and the next quick group will continue the line up.
The nature of the beast is the amount of material in contact with the chamber isn't symmetrical, so the heat is uneven. It doesn't take many thousandths of an inch of barrel movement to make a a significant difference at a hundred meters.
I have no problem with it because I know what is going to happen and can compensate.
IMO it happens to all rifles, some more than others. And this is more speculation than knowledge, bull barrels may slow down the process but it is doubtful they are going to mitigate it completely. I have no doubt that I can measure the movement on a solid block of steel using either cold or heat.
I'm not an engineer I'm a welder and have a bit of practical experience of what metals do when heated-
Barrel flex may not be all of the issue, usually it is a combination of things.
The poster said he tried four different kinds of ammo. Leads me to believe he has been doing a lot of shooting. I'd stick with one type of ammo. let the barrel cool between shots and try to find somebody to watch you shoot that may notice some flaw in technique. Fit is a big thing for me, I'm 6'3" tall and firing a rifle that fits poorly messes with my shooting. Getting the proper eye relief on my scope, a well fitting rifle and constancy of technique were the big hurdles for me, Then I moved on to trigger and ammo and did some fine tuning.
I have what I call a no flinch trigger on my Steyr, a set trigger that you can almost blow on and fire the rifle. I can feel the muscles tighten in my face when I shoot my 7MM Mag from the prone position.
I'm not ashamed to admit I'm a little shy when firing that 7MM while laying down.