MBlock is a part designed for M4/M16 series guns, which supports and reinforces the hop unit and gearbox (Version 2). The gearbox is almost infamous for breaking, especially from the cylinder’s front corners, under too much pressure (aka with too heavy or incorrectly installed upgrade part). The MBlock is installed between the hop unit and the gearbox so that even a cracked or otherwise from the front slightly damaged gearbox is still usable. The reinforcement is not actually a definite solution for a cracked gearbox, but it can prolong the life span of the gearbox considerably and that is why the use of MBlock is recommended even with undamaged gearboxes.
The product is not compatible with two-piece hop units. MBlock is compatible with most manufacturers' M4 and M16 series like AimTop, A&K, A.P.S., Classic Army, DBoys, G&P, Matrix, SRC, Tokyo Marui etc.