Update: King Arms M4A1

Following multiple catastrophic gearbox failures, I’ve finally got my KA M4A1 up and running again.


The most recent failure I’ve had was a cracked mechbox shell – more about the good and the bad with the Deep Fire 7mm Ver 2 gearbox shell(s) after the break.

I’ve been using the Deep Fire 7mm V2 gearbox purchased from work – Jasper put his full confidence into it lasting for my M110 spring and shooting style… however, this one failed prematurely. In fact, it didn’t even make it out to play once before I noticed these fractures.


All 4 corners at the front of the shell cracked – Jasper’s theory is that the moulds are getting old and are not resulting in the cleanest of casts anymore. He did tell me that prior to this batch that was in, the shells were of excellent quality – for a cast monkey metal gearbox, they did a fine job of holding up to much abuse in many client’s guns (mostly thanks to the quality of the casting, being bubble-free, and having the pre-made relief cuts). Note that the rounded relief areas as designed by Deep Fire aren’t exactly round on this gearbox.


I’ve since obtained a replacement Deep Fire shell (thanks to MILSIG’s Head Tech Jasper for hooking me up with a new one as a free replacement – he figured that the one I had was a lemon), redone the relief cuts and polished the surfaces of the high stress areas of the mechbox, and done another shim job. No such issues of micro fractures after gaming my KA M4 for the first time in nearly a year – and I shot it quite a bit as you may have seen in my game play videos from this past weekend… over 1500 rounds on both full and semi auto. This shell has already outlasted my previous one; I suppose Jasper was right in thinking that the previous one was not as great as all the rest.

Other than that, installation of the Deep Fire gearbox was flawless – no major compatibility issues between the shell and the components I’ve installed in there, predominantly SHS and Guarder parts.

“Jester” also installed the same model of Deep Fire shell into his own King Arms… both of us encountered issues with our switches. However, both issues were different – mine was a result of incompatibility between an aftermarket cut-off lever I had just installed and the switch assembly, whereas his was a result of incompatibility between the trigger and trolley. Both issues were easily resolved – neither issue had to do with the Deep Fire shells we installed in the end anyway.


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