The two main types are quad rails and tubes.
Quads look tacticool , but are usually heavier. Plan on investing in rail covers or gloves to protect you support hand or if you dont like chips on your rails. If you plan on having a lot of accessories on your rail this is the way to go. Stay away from utg. Almost everything else is good to go. Daniel Defense makes smoe fantastically lw quads but are pricey.
Tubes are lighter but heat up and cool down faster. Plan on having gloves for you support hand if you are doing a lot of shooting in a short amount of time. Most models nowadays are modular so you can attach rail segments for various accessories. I personally prefer the MI ss gen 2 and Samson Evo/Rainer arms rails. Lighter and thinner.
Either types be sure to check inner diameter and you gas block dimensions. Also check to see if proprietary parts are needed to mount them to prevent any suprises when installing them.
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