My guess is that the barrels are failing from stress corrosion cracking from being over torqued, and aggravated by using a steel, maybe 17-7Ph, that is not forgiving for that mode. A high preload stress from torquing the barrel down tight, pounding from shooting, and presence of some solvents could cause this failure. I don't remember the maximum preload stress for 17-7Ph, but it's about 70 ksi for 17-4Ph, which is a much more forgiving alloy for this mode. The particular heat treat is also a factor.
Are the carbon steel guns failing, or just the stainless?