Just don't buy a 26" space gun barrel with the thought of lopping off 6" to make it 20" and hit the ground running. The extra 2" longer gas system won't like that and you'll end up with a soft shooting rifle that won't function well with standard pressure 55gr ammo. Ask me how I know....
It can be made to work by opening the gas port from .100" to .110-.125". Might be able to play with reduced weight BCG as well. I only went to .110" and it worked perfectly with everything but the crappy Wolf Polyformance. Barrel reportedly had 2800 rounds on it. Shoots 1 MOA with 55gr ball and 1/2 MOA with 75gr Horn HPBT handloads. Not bad for a $75 barrel with the gas block and tube. So, it can be worth the effort if you can do the work yourself, but much easier to start with the 20" service rifle barrels. I thought the balance was perfect with no additional weight and an A2 buttstock.
