So, you're saying that the barrels installed on .357 Magnum revolvers are stronger than those put on .38 Special guns? Exactly how is this done? Are all magnum barrels of larger diameter, or are they made of different materials, what's the scoop?
And I still maintain that the cylinder takes the brunt of the expanding forces, not the barrel because some of the pressure is lost in the barrel/cylinder gap. Also, by the time the bullet is in the barrel it is moving pretty fast and accelerating. This means the pressure inside the barrel is escaping much faster as it is pushing the bullet down the bore, and doesn't put nearly as much strain on that part of the gun as it does on the cylinder when the cartridge is first fired since all that energy within the chamber is pushing in all directions and meeting mostly solid resistance from the chamber wall.
Think about it. The powder charge ignites, and there is an instantaneous pressure surge. The bullet is forced from case and jumps across the cylinder gap. Some of the pressure pushing the bullet escapes through the gap. It seems to me that given the pressence of this gap there is no way the pressure within the barrel can even approach that within the cylinder. Then as the bullet moves faster and faster down the bore the area containing this expanding pressure gets larger and larger since as the bullets goes down the barrel it creates an ever-increasing space behind it for the pressure to fill.
Even at double the chamber pressure, I think a barrel that would work for a .38 Special would be OK with a .357 Magnum. I would note that in my own experience I have loaded .38 Special ammo to very near Magnum pressures for use in heavier guns. So far the barrels have not been a problem. Also, in my 34 years of shooting and being around shooters I have never seen or even heard of anyone rupturing a barrel with excessive pressures. I am seen a number of cylinders let go, but never a barrel. As I recall when S&W built the prototype for the M19 in the mid-1950s it used the same barrel as the M15. Bill Jordan and others urged them to go to a heavier barrel to add weight.
Of course, there's one way to find out. I have a Heavy Duty .38 just like the one pictured above. Someone send me a non-recessed cylinder from a late model M27 and I'll slap it in there and give it a go. Assuming it's the same length, of course. I have some loads on hand that should definitely do the barrel in if it's inclined to go.