If you have an AR-15 sitting, rusting in the safe, you could try a conversion kit to make it a straight-pull bolt action.
This is not what the AR was designed to do, but it gets the parts outside.
I converted a Norinco M4 clone with a Spectre Ballistics kit and my neighbour converted his Smith & Wesson M&P with a Maple Ridge Armoury Renegade kit.
They aren't semi autos but cycle fast and well and the MRA version has the benefit of a spring return. Both worked well with no problems.
Again, not a semi-auto but light and can take down for back-packing.