Will the FM15 upper function with a lower that has a standard buffer tube, spring, and buffer? — 1 Answer

Yes it will but there is no need for the buffer or spring. The bcg and internal recoil springs keep everything in the upper receiver. This gun can be fired with the stock folded, or no stock at all. If you prefer the stock that go on buffer tubes there are quite a few skeletonized buffer tubes that can have a standard stock installed on them and can work with any folding mechanism you prefer. These uppers and guns are very light, very soft recoil and being able to fold it up for storage or getting in and out of vehicles and being able to use the weapon with the stock folded is an excellent advantage over the fixed stock or folding adapters that can’t be fired while folded.