38 super was simply the 38 acp loaded to what was probably the first "plus P" level...way back in the late '20s (1920s). You have to use 38 acp/38 super dies. It has a slight rim and headspaced on the rim although some gunmakers headspace the cartridge on the mouth. Easy to load, much better than the 9mm P, and I have found it a little more accurate -- for me, that is. Takes heavier bullets than the nine and is available in some darn good guns. It got displaced in history by the advent of the 357 in the 30s but a lot of us still like it. Funny thing....a couple years ago, one of our club members bought one, made in another land, for bowling pin matches...figuring he could shoot it in minor class. He called me asking why the gun manufacturer stated he should not use +P loads in it. Kind of funny when you consider that the super is a +P from the get go. (or it was).