As I understand things, there is considerable resistance when the bullet first enters the barrel. If the load is not strong enough to overcome that resistance, the pressure builds up and can blow the cylinder. This can happen to rifles, as well, and the problem only began to be seriously investigated after a number of C.A.S. guns blew up shooting mild smokeless loads. People are always saying to shoot smokeless in old guns, it's perfectly safe, blablabla..... As I've said before, there is little margin of safety in these old guns. Bugger up a load by using a magnum primer instead of a standard primer, or have a slight overcharge due to a sticking powder measure, and you could bulge the cylinder on your fine old Colt, or worse. These guns are irreplaceable, and are more than just a fun gun to shoot. They are part of our history, and we are the custodians of that history. There is no worse feeling than causing damage to something that has survived intact for 130+ years! I nearly killed my friend after he gouged the stock on my old Martini-Henry. Anyway, I keep hearing about how BP is "corrosive", it will "pit your gun", "damage the bore" etc. etc. If it really was as bad as these guys claim, there WOULDN'T BE shootable antiques around today! Many antiques do have dark, pitted bores, but these were smokeless era guns, damaged from mercuric, corrosive primers. As for the full auto SAA, that happens when the recoil plate around the firing pin is missing. When the first round is fired, the primer flies out, cocking the hammer. The whole cylinder will go before the shooter can react and release the trigger! A fellow in the US got in trouble, decades ago, when he fired a shot into the ground in front of a trespasser. The gun went F/A and stitched the trespasser, killing him, as the gun climbed under recoil. On testing at the crime lab, it was proven that the gun did indeed fire full auto, just as its owner claimed it had. This could also happen with early .357 magnum SAA's, too, I believe, as they used the same firing pin as the .45's. It could also happen with the .38-40, which is, I believe, the reason factory primers are crimped in now.