I think the mid length is the best compromise for the 16" barrel. I have an armalite 16" mid length and its as easy to shoot open sights as my full length A2, compared to my CAR upper. Eye balling my gas tube it looks like its about 11 3/4" or a little less, and the gas port is about 10" from the back of the barrel extension if that helps. Unless its a specialty item its probably the mid length setup. I have the same gas block setup on my bull barrel and you got it right. If its the 2 setscrew type thats all that holds it on. It should fit on snug so it doesn't leak gas. I used a block of oak to help drive the gas block into position. Make sure you test fire it before cranking down on the setscrews to keep them from wanting to return to where they were first set. My buddy puts locktite on the bottem half of the gas block (to keep it out of the gas port) for a little added security. I don't have anything mounted on my gas block rail so I didn't locktite it and I've had no issues with it.