Another thing I like about GBBs, is its more realistic in bad conditions. You get dust/sand, etc in there, and the cocking handle starts to slow down, jam, etc, while the gearbox is sealed.
If you have a jam in the gearbox, you often have to take the damn thing apart, but on these, riding the charging could solve it...
Would be fun to play in dirty environments and see how these hold up. I bet the M4 will jam because its an M4
Cheers,
Alex