Quoted:
More experienced voices will come, but the consensus thus far has been autoloaders need to be full length sized, and unless you're reloading for your bolt gun with brass from the same bolt gun, it also will need full length sizing. Firing the brass shapes it to the chamber in which it was fired. No two chambers are completely identical.
I agree. I recommend full sizing for auto loaders. You an do neck sizing for your AR, but it's a little risky, especially if you are new to reloading. Without full sizing you may run into rounds that won't feed from your mag, a case getting stuck after firing, or even worse a round not fully locked into battery AND it's stuck and you can't pull it out.
Be sure and check your case length after you full size.