Hi mate..
The hop up is internal this requires inner barrel removal, on this weapon we've been lucky in that it's already pretty much spot on (well not enough out to really bother about).
The weapon is fully stripable as per a real M4 variant, I'm not quite sure what you mean by "lower part of the rail", if by that you mean the lower receiver, then yes it does, if you referred to the lower rail (RIS) then no it isn't.
What I really like about the RIS config is that it removes absolutely any possibility of wobble between the rail and weapon body, this thing is rock solid.
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