4. If you use a lot of different loads in the same AR15, like supersonic and subsonic 300BLK, with and without a suppressor, then you can end up very overgassed or undergassed getting them all to work. For example, in my carbine length 300BLK the action will not cycle subsonics unless the suppressor is on. It works fine for supersonic ammo (BHO). So for subsonics, it is undergassed. But there is an easy fix for being undergassed... It might be a little scary, but it is very easy to drill out the gas port on the barrel. It only takes a few minutes with a drill press (or very carefully with a hand drill) to open it up. A quick tip... put a small dowel in the barrel to prevent the drill from going all the way through the bore and hitting the rifling on the other side. Try going up in 32nd inch increments if you have a drill index. A little bit makes a big difference. Once it is drilled out you can add an adjustable gas block and will be able to use a wide variety of loads with just a gas block adjustment. If you are undergassed, adding an adjustable gas block only will not help. They are only a way of REDUCING gas flow, not increasing it, so you need to be a little overgassed for it to work properly.