With the Covid-19 situation resulting in a lot of extra free time, I decided to fully rebuild my MK46 gearbox and do some more work on the externals... Figured if I get that effective range up I can social distance while putting rounds down range... right
So first I reshimmed the gearbox and swapped out my piston for an SHS lightweight one along with running final checks on my cylinder volume vs barrel volume. Credit here for a great guide on volume matching:
Link. So after building an excel chart for future use and plugging in the numbers off my build, a 380mm barrel is perfectly matched with a hair of overvoluming on the cylinder.
After that, going through and checking AOE on the piston I noticed some strange contact points. Turns out the SHS piston with its larger pickup tooth actually contacts the portion of the sector gear that meshes with the spur gear. So out comes the dremel to trim that down a bit, after which the piston was still binding slightly. So after some further investigation the contact point is the pickup tooth again meshing with the leading tooth of the sector gear. So after trimming that down a bit, the piston and gears mesh beautifully and the gearbox is running superbly. ROF, unsurprisingly, increased as well.
After getting the gearbox all sorted I turned my attention to the stock buttplate again. It was meshing well, but tight in a couple spots. So dremeled off the one edge completely to free up some more room for wiring, after this no more pinching! Lastly, the plate didn't like to line up with the upper body pin holes. It would work, but wasn't properly aligned and somewhere, something was getting twisted to make them align once the pin was tightened. Using the dremel I enlarged the lower body pin hole on the butt plate and after taking off a small amount of material, everything lined up nicely like the stock CA one did.
After that while reassembling everything I noticed some resistance on the barrel retention clip. After taking a closer look something was certainly off, it has significant resistance towards the ends of the arm travel and the wheel that secures the barrel in place was jammed, with obvious signs of wear from where it meets the barrel - evidence this was a long term issue. The body pin holding the arm in was wedged and had to be pried out (can't get a punch on the opposite side). After taking a look at how everything tied together, this arm was either damaged at some point (took a spill while running up a hill summer 2019, might have done it) or it was always this way and only now had dug into the body enough to catch. I did try bending the 'tabs' to be aligned but this proved useless, further suggesting it was this way from the factory. Either way the angle of the pin was about 95 degrees and this was forcing one side to grind against the gun body. By dremeling out a little bit of material from one side the pin was able to align and now it holds the barrel in very securely with no resistance on arm travel. The first picture below shows the wear on the gun body from where the catch was contacting it.
