I'll try to put it in a nut shell best I can. Working metal into any kind of shape imprints stress on it. Each time you fire a rifle the barrel and action absorb the heat from the round going off and the friction of the bullet passing through. So the more you fire it, the hotter it gets (youtube has plenty of videos of guys lighting the wood stocks of AK's on fire from pumping several hundreds of rounds through them as fast as they can). As heat builds, the stresses in the metal will cause some areas to contract, and some areas to expand as the metal tries to return to it's original shape. Causing the barrel to deflect as it warms up and throw your point of impact off.
Lets say you zero your rifle on a nice sunny summer day when it's 95 degrees out. Take that rifle out in deer season when it's 20 degrees and depending on the stresses in the metal it may be off at even 100 yards by an inch or more.
You fire your rifle 20 times zeroing it one day. After several shots and the barrel is nice and hot, and
that's when you've zeroed in your sights... Next day you take it out and the first shot out of a cold bore might be off. You get the idea.
Almost NO manufacture is going to waste time and money cryo treating because some have tried, and it's an expensive process for them because its time consuming and slows production DRASTICALLY.
Worse yet, the customer can't "see it".
If company A makes a rifle that's "more accurate because it's got special gold speckled paint on it", and company B makes a rifle that's "more accurate because it has cryo treating", customers have proven they will buy the one with gold speckled paint and swear loyalty to it like it's a religion because they can see it. Which means you can brag about it, which means "it's better". Stupid, but we all know it's true.
Bull barrels work! The thicker the metal, the longer it takes to heat up, the more resistance it
commonly will have to deflection (depends on what random stresses may be in that particular metal. COULD deflect MORE once eventually heated!). More importantly they "feel solid" and you the shooter / buyer
SEE the difference. So you have confidence in it, and your shooting actually WILL improve with confidence in the weapon. Proven!
Bull barrels
IMO suck because most obviously,
their heavy. Also because they take farrrrrrrrrr longer to cool once heated up. Their
heavy add stress to the receiver of the weapon. And it bothers me personally carrying one because in my mind I know
primary reason it exists (as a marketing tool / "Idol"). THAT'S JUST ME THOUGH.
SO HATERS RELAX!!! They DO work! As stated.
Cryo treating is a process where you freeze the crap out of the metal (putting it in a nutshell

), relieving
nearly all stresses in it. No stress = no deflection. I've got half a dozen rifles in everything from .22lr, to .308 I've done testing on and cryo treating has proven effective in every one of them. Three of them DO have bull barrels, and it still improved them. One of them DRAMATICALLY. It again depends on how much stress is in each particular barrel. So results will vary. I've had receivers treated as well and am happy with the results.
Google: "Cryo treating barrel" for more info.