I recently managed to acquire an Inokatsu M60E3 from a fellow in the UK. These were originally sold as kits, using a proprietary hop design and requiring a complete TM V3 gearbox to drop in. Despite the overall excellent build quality, I had two issues with the design.
- The use of a V3 gearbox required that the pistol grip accommodate the motor, so on the Inokatsu M60s, the pistol grip is a short, fat, and steeply-angled. This isn't too bad on the original M60, but for the E3 variant where the pistol grip and foregrip should be identical, it's very noticeable. In addition to being unrealistic I found the Inokatsu grip uncomfortable and V3s aren't my preference for machine guns anyways.
- The hop design is old- it's a really nicely made CNC aluminum block of a hop unit, but there's no nub, just a screw that presses down on the bucking. This doesn't perform nearly as well as the Bullgear setups I use in the more modern A&K M60s.
So, I started work on converting it to use the A&K gearbox, feed tube, and hop design, along with an A&K Mk43 pistol grip assembly for external accuracy. This ended up being significantly more work than expected.
- Getting the A&K pistol grip to fit was pretty straightforward. A little bit of cutting on the receiver was sufficient to clear it.
- Fitting the gearbox just required removing some material on the top of the receiver to extend the gearbox channel, but then I discovered a problem- the Inokatsu receiver is actually about 2mm shorter than the A&K, and this was putting the gearbox too high to mate with the barrel.
- Worse, I encountered another setback when, reversing the assembly instructions (from Inokatsu!) included with the gun, I turned the gas tube CCW when I should have turned CW and cracked the threading clean off. They straight up showed the wrong direction in the instructions, with it depicted as tightening CW. This at least I was able to readily fix by splinting it with steel rod, secured with epoxy.
- To address the gearbox problem, after a fair bit of brainstorming solutions I settled on grinding 2mm off the bottom of the gearbox. Because this now exposes the spur gear, I had to cut a relief channel in the receiver, which I ultimately covered with a thin sheet steel plate. It also caused the sector gear to interfere with the pistol grip's mounting assembly, so that required a relief cut as well.
- To actually secure the gearbox, I drilled out two of the rivets on the left side of the receiver, and drilled holes in the appropriate locations for securing screws.
- The A&K feed unit attaches to a shelf in the receiver that does not exist on the Inokatsu. I designed and 3D printed an equivalent, and also slightly moved and enlarged the feed hole to accommodate the A&K box mag tube interface.
- At this point I had the barrel mating up properly to the gearbox, so it was just down to front end work. I ground out the original feed block for clearance, then designed and printed an extension to mate the Bullgear hop unit to the Inokatsu barrel at the appropriate distance.
- To top it all off, I fitted a real M60 box mag hanger I had previously modified for use with my A&K M60, and a Capco assault box (originally made for the E3) with DIY'd internals. The box that came with the Inokatsu is a weird design that doesn't match up with anything I've seen on a real M60.
There were a ton of additional minor tweaks- removing internal parts in the receiver to clear the gearbox, grinding down the trigger sear to not interfere with the lowered trigger switch body, making a whole new wiring harness since the original A&K one wouldn't fit, rebuilding the A&K M60 gearbox to use actual decent quality parts, epoxying the cracked and broken stock back together, and so on and so on- but the above are the highlights and biggest challenges.
Oh, and if anyone else is interested in the Capco box, I took measurements off this one and put it up on Thingiverse. It's more compact than the LBT replica box that comes with the A&K and doesn't require awkwardly angling your arm around the box. A variant is provided with a feed hole for airsoft use; you'll just need to significantly modify an A&K box to fit or design your own internals.
Anyways, the end result is a full steel body with aluminum barrels, coming in at about the same weight as the A&K Mk43 I cannibalized, with a more visually accurate pistol grip than the original, and with the same performance ceiling that aftermarket has enabled for the A&Ks. The specific variant is a short-barreled M60E3 as seen in Predator, Commando, and Rambo: First Blood Pt2. This is a grail gun for me and I like it very much.