Regardless of whether you think an AR-15 pistol is pointless or not (I use mine as a PDW for home defense), an adjustable gas block is a necessity. (this applies to AR-15 rifles as well)
The problem with AR-15 pistols is too much gas pressure traveling down the gas tube due to the shorter barrels which results in a lot of recoil. Too many times the "solution" is to address the symptoms rather than the problem. I see many opinions that in order to tame the recoil, one should resort to a modified buffer coil, a heavier buffer, a pigtail gas tube, etc., rather than addressing the problem itself...too much gas pressure.
How do you solve the problem of too much gas pressure? Simple. You limit the amount of gas going into the gas tube. How do you do this? Simple. Install an adjustable gas block which can be tuned to limit the amount of gas going into the tube, which allows only the necessary amount of gas needed to cycle the bolt, eject the empty shell casing, and chamber a new round.
There are quite a few adjustable gas blocks on the market now. One only needs to Google the term "Adjustable AR15 Gas Block" to find a selection of products.
Before I installed one, my AR-15 pistol kicked pretty hard. Harder than my AR-15 rifle. It was not very pleasant to shoot and suffered from harsh recoil. After acquiring and installing an adjustable gas block, one could easily be mistaken that they are shooting a .22LR version instead of the 5.56 version. Yeah, it makes that much of a difference.
Some adjustments at the range to find the ideal setting that works well with soft shooting bullets and hot loads is all it takes to ensure that your AR-15 pistol runs reliably and with minimal felt recoil. The same can be applied to the AR-15 rifle, especially those running a carbine gas length systems.
However, an adjustable gas block isn't going to do anything for the fireball issue out of a short barrel.