There is more to it than just the length of the gas tube. In fact, it probably has the least to do with it. The real factors are the dwell time and the size of the gas port opening in the barrel. The longer the bullet is in the barrel past the port, the more pressure you have coming down the tube. The bigger the gas port opening, the more pressure you have coming down the tube. The others who use them might. E able to share more about the benefits of an adjustable block, but most AR’s shouldn’t need one if all else is in check. Gas and recoil-wise.