When a barrel is dented, the outside sees both stretching and compression. Then inside sees both as well, just in opposite places. The crease of the dent sees compression on the outside of the barrel and stretch on the inside. The area around the dent (rim of the crater) sees stretch on the outside and compression in the inside of the bore. Basically, bending.
Bulges are all stretch for all practical purposes.
When you beat on a barrel over a mandrel, it's like pounding pizza dough, if you hit it hard enough and enough times it will thin and grow in width and length. The trick with dents is to not hit it any harder than needed to undo the bends, so it doesn't stretch the metal from over compressing it.
With bulges, you are trying to shrink the metal. It can be done. Some bulges are larger and take special techniques and are therefore, buggers. Slight bulges don't take a lot of effort to shrink or compress the metal.
