I observed the advantages of using a rifle sling during a trip to Northern Rocky Mountains Provincial Park, British Columbia. A good friend and coworker swore by these holsters and over the course of our trip, I'd watch him use poles and scramble up steep rock without fussing with his rifle. Most days were spent hiking uphill and I often found myself opting to cradle my rifle—this required one or both arms—rather than have its conventional sling constantly slide off my shoulder.
After receiving the materials, making the first sling took roughly an hour. And most of that time went to considering how it would get put together. The rifle sling is composed of two parts, a holster-type piece attached to the hip belt of your backpack (holds the butt end of the rifle) and a small attachment point for the rifle’s barrel, placed on one of the backpack straps near or at the sternum strap. The large holster piece supports the vast majority of the gun’s weight, while the barrel strap prevents it from jostling around. The barrel strap is a short piece of webbing that’s velcro’d to the backpack strap. A section of this webbing (~10 cm) passes around the rifle barrel before entering a camming buckle. Unholstering the rifle is quick: you flick the camming buckle open, the forend of the rifle falls into your hands, and the butt end of the rifle lifts from the holster as the gun’s shouldered.
Tip: I found that it is important to size the rifle butt piece carefully to avoid your recoil pad getting snagged when you unholster the gun. Selecting a finely woven webbing appears to help as it's less likely to bind on a soft recoil pad. Also, I find that using a camming buckle larger your webbing’s width makes it easier to thread the webbing through on account of the additional clearance.
For those interested, here's a materials list:
2 x 26" pieces of 1.5" velcro (hook and loop; lower 1/2)
3 x 9" pieces of ~2" webbing (lower 1/2)
1 x 9" piece of 1.5" webbing (lower 1/2)
1 x 3" piece of 1.5" webbing (lower 1/2)
1 x 1.5" slide buckle (lower 1/2)
1 x 13" piece of 0.75" webbing (upper 1/2)
1 x 0.75" cam-locking buckle (upper 1/2)
2 x 9" pieces of 0.75" velcro (hook and loop; upper 1/2)