It's a HurricanE 7mm Reinforced Version 3 Gearbox. I found out what was causing it, it was the peg that goes into the hold on the side of the cylinder head. I just used a small flat head screwdriver and was able to wedge the peg in, and then I was able to completely close the gearbox.

Your King Arms gearbox probably isn't defective, when you received it, wasn't it closed up in the package or were the 2 halves separate?

It came closed up in the package. It did somewhat close together with the cylinder head installed, but it did not close properly. It actually rotated the cylinder head forward, causing the tappet plate to jam and the cylinder to sit improperly in the gearbox. I returned it and got a G&P, which had other issues, but were fixable.

Especially when you're working with a hot and cocked mechbox. Those are always fun.....

gotta love the modify modular gearset. comes with a slotted anti reverse latch that you can release it with. :p

Use a long allen key or screw driver shoved into the hole in the spring guide, use that to hold the compressed spring while closing the mechbox, and angle it down to seat the tabs on the guide into the mechbox shells. Line up the gears and bushings (I always, where possible, put the bushings on the gears beforehand, much easier to line up with the mechbox holes). In order to fully seal the two halves together I'll press the nozzle a couple times, often I'll find the tappet or cylinder head is holding it open a bit, this usually lines things up perfectly (I find this especially useful on V3 mechboxes) as well as makes sure the tappet and nozzle are working properly too.