The M655, a distant cousin of the M16 and CAR-15 has been released by G&P. The real steel version has been sold to the American Army, Navy and and Air Force and is one of many variations. It has also featured heavily in a game called Metal Gear Solid so if you like dressing up as Snake, then this is something you can use as a Primary weapon.
The parts that you'd expect to be made out of metal, such as the receiver, front iron sight and outer barrel are made of Die-cast Aluminium and everything is is made out of plastic. Of course, all these parts are easily replaceable with aftermarket external and internals parts.
The hop up can be found as you pull back the mock charging handle. The carry handle is apart of the upper receiver but can be switched out if you wish to have a top rail instead. At the back we have a stock where the battery is stored. Of course, G&P have many of these aftermarket parts. This may be the beginning, but it'll most likely be modified to suit your tastes!