The blackhawk Ruck Pack uses an Alice frame, kidney pad and shoulder straps. I got the Ruck Pack Kit which inclides the frame and such. It's packed right now as a Winter 72 hour kit. In the spring I'll move the frame over to my Medium Alice pack for lighter weight over nighters and 2 night trips.
Alice equipment including a webbed belt, butt pack and Y-harness or H-harness use Alice clips to hold things together. In other words the buttpack attaches to the webbed belt via alice clips, ammp pouches clip to the belt or to a pack with alice clips. It's a modulare and rugged system, but kind of heavy compaired to modern backpacking packs. My Blackhawk Ruck is 9.5lbs with the two removable pouches, is 6,000 cu/in and can handle up to 100lbs. Thats better then a lot of hunting packs that I've seen.
Along with my Alice pack and Blackhawk ruck, I use a Surplus Webbed belt, buttpack, H-harness, Canteens and Canteen pouches and M-16 mag pouches to carry light loads. Sometimes I've used this setup for overnight trips and as a BoB. It's my standard Day hike setup. If you can find a copy of the Alice gear manual from the 70's (mine is from 1974) it will have lots of info on seting up at attaching things to the gear.
Get an Alice pack with frame for cheap or another compatible bag and the options to customize it are many! If you have more questions, feel free to ask or email me.
