There are 2 major types of gas systems that the AR runs on. The first being DI (direct impingement) and the second being piston. DI is the original design. Piston is a relatively new development. There have been forays into piston designs in the past, but it’s only been recently that piston driven ARs have been become popular. The best thing I can do you is show you the pros and cons a piston system offers, it’s your personal choice to decide if it fits your needs. Please understand that since there is no set standard for a piston system, every piston system is a proprietary design that will require different and non interchangeable parts.*
*A cleaner running system. (less fouling of the BCG and chamber, more fouling by the gas block or exhaust)
*Should be more reliable in adverse shooting environments.
*Should be easier to clean, (provided you don’t need to remove the hand guards)
*Costs more than a comparable DI setup
*Weighs more than a comparable DI setup
*Harsher recoil impulse (I concede that this is subjective and debatable)
*Inherently less accurate,(minor)
*Slower follow up shots (minor)
*Proprietary parts make it harder, costlier, and longer to replace broken parts.
Personally, I would only recommend a piston setup if you where looking for a non precision type carbine and need the extra reliability due to your expected shooting environment.
DI gas systems come in 4 flavors: Rifle, mid-length (middy), carbine, and pistol. The most common types are carbines, with 7” handguards. The next most common types are rifles with 12” of rail. Relatively new developments are midlengths with 9” handguards. Pistols are uncommon and I won’t be going too much into them. Only consider rifle length gas systems for 18+ inch barrels. Both carbine and midlength gas systems can be used for 14” to 16” length barrels. For these lengths, I usually recommend midlengths as their advantages outweigh their disadvantages: