Yeah, some of the terms used around here can be confusing to new shooter, heck sometimes us older guys forget what they mean too. lol
As for Open, under the USPSA rules it can be any gun with a bullet diameter 9mm/0.355 or greater and a magazine no longer than 170mm. Other than that pretty much anything is legal. But the popular deffinition is a gun with a compinsated barrel, red dot or other type of optics, high capacity magazines. Something like this...
Joe W.
Thanks and well put SLM,
@ Joe, your "Open Gun," is beautiful! Lol..