The m4 gearbox will not hold up with sustained fire. It is prone to cracking and will crack over time no matter if you radius the gearbox or not. A gearbox built specifically for an lmg is far superior in this respect. They are much thicker and more durable by leaps and bounds. Also, just adding a box mag to an m4 does not turn it into an lmg in any aspect. It's still just an m4 but now it has a big magazine.You can actually buy a high capacity motorized magazine for the M4 that will basically make it an LMG. I do have an A&K M249 which is a blast to shoot but it is heavy at 16LBS. I also am not real impressed with the magazine that comes with it. But the gun itself seems to be solid and has worked flawlessly so far. I have read that the gearbox in them is heavier duty for sustained full auto fire. I have also read that the A&K is considered the best by many except for their magazine. Which the magazine on mine is very loud and sounds like a rock crusher. it also has trouble keeping up with long full auto bursts, after 30-40 seconds it will misfeed I have to let off the trigger for second so it can catch up.