ok so this is exactly how much height difference there is between a metal m4 body and the 416's front end. its quite a bit and also the 416's receiver had a bar coming out of the top rail to keep the front ris on straight, now its wobbly. as you can see its a g&p zombie killer body, everything was a very tight fit. there is barely any room at all for the inside bolt to slide back. im thinking about taking it apart and just removing it. the bolt cover doesnt have a latch and fly open when you pull the charging handle which is kind of disapointing along with my other gripes about the metal body. all my m4 mags are jg's and are very wobbly in the magwell. if you own a 416 i probly wouldnt reccomend getting a metal reciever for it as mine just doesnt really look like a 416 to me anymore unless you can find a lower metal receiver and be satisfied with your upper being plastic
things i had to dremel: the hook on the charging handle in order to fit the spring, the U shaped slot on the fixed stock, the nub that holds the bbs in on my mag in order for the mag to lock into place in the magwell
some of the body pins are pretty difficult to install, the selector switch is pretty stiff moving and slightly crooked, and the thing above the mag catch is pretty loose
p.s. it came with a metal hop up which wasnt nearly as difficult to assemble as id imagined