I had a G&G M4 blowback come in for repairs and a checkup. This one was originally from Mach1 Airsoft....I don't know first hand the difference between the one they sell (higher end of the clear receivers) and the others (i.e. lower quality ones).
Pneumatic system:
- quite an ingenious design. There is a simple hole in the cylinder at the front near the cylinder head that allows air to get pushed up into another small cylinder riding piggy back on top of the mechbox. The piggy back cylinder is about the size of a pencil (people still use pencils...right?).
- The piggy back cylinder has it's own piston that is forced back under pressure, until the piggy back piston head clears a vent, where the air escapes and the system depressurizes (the bb has probably left the barrel by then as well).
- the holes between the piggy back cylinder and the main cylinder are sealed by a rubber grommet/gasket. It's quite soft...so take care not to nick/tear it.
- at the end of the piggy back piston shaft...there's a c-clip that holds the fake bolt. Under pressure, the bolt is pushed back...with lack of pressure a spring snaps it back forward. It's not so hard that if you stick your finger in it that you'll think you'll loose your finger tip (tested that...
).
- it travels back about half the length of the complete ejection port.
- unlike other "fake" dust covers...this one straddles the mechbox and rides in a slot on both the right and left sides of the mechbox (the slots are actually on the piggy back unit).
Internals:
- pretty typical stuff. very "sportline"-ish. Solid and tidy. Thumbs up...nothing a little TLC can't make better.
- big bronze bushings (9mm?)
- shimming was extremely poor, greasing/compression was decent
- spring guide is metal...but the plain, no-bearing type
- piston head is poly...with the "tornado/vortex" type ports
- piston is poly...2nd tooth already removed.
- AOE was a little early
- wiring/solder was good
- the spring release forward assist was a great touch...it works well
Externals:
- sportline-like...nothing great, nothing overly poor.
- the upper and lower come apart like a "CA"
- putting back in the front pin is absolutely horrible. A LOT of "gentle" force was needed to get the front holes even remotely lined up. The pin fit is extremely tight throught the lower. I can see a lot of broken lowers from guys monkeying around with their AEGs.