I've mentioned it before, but with the USMC transitioning from ILBE to FILBE (or USMC Pack whatever name they land on) they're surplusing the ILBE cheaply. The 3 Day "assault" pack is under 100 new, even less used (though "used" covers a LOT of conditions so I go new), has a thicker shell/fabric than most packs we can get, PALS webbing, a spot for a hydration system (that may or may not be wise in an indoor range for lead reasons) and works well/mates with the main pack good for a bugout bag or backpacking.
It gets slightly better reviews than the MOLLE II system with a buttpack that does abou the same. Plus I don't think the buttpack has PALS webbing.
Military Surplus will get you a tougher/heavier duty item than you can get on the consumer market for cheaper. If you don't want to look like you're playing dress up, then dye/paint/cover it.