For military use it's typically a drop leg holster that goes with all the battle rattle. For an ordinary holster you will probably have to alter your holster positioning slightly. Probably 3 o'clock would be the most rearward position you could obtain depending on your build and how that pack fits you. I'm odd though, I prefer drop legs anyhow. If used properly they are simply more comfortable, efficient, etc. They just don't fit into every day civilian carry/life very well.Doh! I just realized the pics I posted in the group buy thread were supposed to go here.
My question at the moment, though, is how to best add a handgun holster to the rig. Would you wear a tactical vest under the pack? I would think the pack's straps would interfere with most of the pouches on a tacvest. Would you appendix carry an OWB holster on the waist belt? I prefer a 3 or 4 o'clock carry, which would place the sidearm on the outside of the padding (or inside the padding if I was wearing the holster and then put on the pack).
Standard loadout is plate carrier with full IFAK, at least 3 double mag pouches, plates, and grenade pouches. They can also have pistol mag pouches if they fit and/or dump pouches. On top of that you'll have the complete ILBE system (main pack, assault pack, camelbak). Then any additional equipment depending on their role/responsibility in the unit such as ammo, spare barrels, HEDP 203 rounds, etc,etc.
For the most part you don't go into a highly hostile situation with full main pack. If you're expecting confrontation or a higher than average chance of it you'll be utilizing the assault pack. So, yes you'll have your vest on when using the pack, but the chances of you needing to access your magazines is less likely. They are still accessible though. You make sure you setup the vest where everything functions well together so you are not hindered in any way from being able to engage at any time.