I use a Viper 3 point on my MP5 and my L85, works just like the real issue L85 sling.
There's a clip on it that you can fasten and unfasten for more or less mobility. When it's fastened the gun is held against your chest, allowing you to walk around with it attached to your front, climb ladders etc etc. When it's unfastened you can move the rifle with ease and point it in any direction with your shooting hand, so long as you keep the stock relatively close to your shoulder, you can also drop the gun to let it hang at your side as if you were using a one point sling.
The only downside is that I don't think it will be anything like long enough for anything larger than an M4. Too small for my M16, plus the M16's sling swivels are just in a retarded place so it wouldn't really work anyway.
So in short, I just wasted my time writing this lol.
I'd just get a standard two point sling if I were you, just so you can sling whichever rifle you're using over your shoulder when walking to and from the safe zone, it'll at least take the weight off your arms for a bit.
My L85 weighs about 6kg with all the stuff attached to it, if I run without a sling then I can barely move my arms most of the following week.