To clarify (possibly) what I said before, cryogenics is most beneficial to refine, and complete the heat treat process in steel. The crystaline structure of steel changes from austinite to martinsite (sp on both may be wrong) when the steel passes through it's "critical" temperature range in the cooling step of the heat treat process. Somewhere around 85-90% of the austinite is transformed. When the steel is taken to very cold temperatures ( like -100 degrees) that transformation is nearly completed say 98%. This makes the heat treated steel so treated more stable, and the heat treat more uniform, but does nothing for steel in it's annealed state as a 4140 barrel steel would be. The two most common stress relieving processes that I'm familiar with are vibratory, and heat with heat being the most thorough. I've seen one place that advertises stress relieving your firearm without disassembly......... that must be cute. 
Now to the knifemaker that buries his knives in a snowdrift...... he has the right idea, and it will help more than hurt his knife blades, but to do a good job of it he should alternate the cold with warmth for a few days, or weeks. That will speed up the transformation of the crystaline structure to the martinsite. When I was making tools for my own use, or ones that I had time to spare on, I would hang the heat treated steel out a window for a few months. When I worked at a place that had a freezer capable of going over -100 degrees, we used that. By the way, that freezer was used to stabilize heat treated bearing races that were gointo be used in space.
Good explanation Tallwalker.
The Cryo treatment helps most with the more advanced steel. Generally, the more complex the heat treat, the more cryo helps. The steels that require long soak times seem to especially benefit from it.
The faster air quench steel like D2 also benefits from it.
My favorite and the one Bruce uses the most is a simple carbon steel, 5160. I've called it the king of steel, because it's so forgiving.
Many cartridge barrel makers still use it for the same reason. Some have switched to a 10xx series but it's close in the HT department. Cryo helps a tad with 10xxx which is why most car makers have switched to it for springs...but the same Cryo treatment does nothing to further refine the grain in 5160 (Which I suspect the Handi Barrels are made from).
We've tried numerous things on 5160, true Cryo (Not in a snowdrift
). I spent months carting liquid Nitrogen around, sent them out for treatment to a lab....I've used multiple quenches, cure times ranging from hours to weeks between temper cycles, etc.
There is very little change in the structure over the tried and true, harden and temper method. The stuff is bombproof as are many carbon steels. That's why it's so popular for everything between knives to farm implements.
The snow drift started as an experiment and then became a selling tool. It does not produce a better knife though.
The carbon steels should be relieved after either forging or grinding, but that's better done by one or more annealing cycles. Nothing worse than having them curl up in the tempering oven. I may relieve one ten times or so before heat treating.
I use carbon steel for gun springs in the locks I build and I've tried cryo on them. There's still no noticeable improvement over harden and temper.
The barrel makers I know, use somewhat softer steel and switch around some. Mark DeHaas doesn't stress relieve his barrels at all and and I have never had one of his string shots. I have seen some barrel makers "Adjust the barrel" in a tree fork
, but the only stress that relieves is making the barrel shoot straight and makes the maker feel better.
As far as I can tell, Cryo is suitable mostly for industrial uses like tool making and then only with more exotic steel formulations.
