This will focus on the gearbox upgrades I did to upgrade it from 435fps to the field maximum of 475fps. The bump in power should give me an additional 20 feet of effective range. Meaning I can hit a torso at 190ft, nine times out of ten. (As an aside, anyone that claims they have an effective range at 300ft is a liar, because math. Check out the Airsoft Trajectory Project @ http://mackila.com/airsoft/ATP/ to see why.)
The AUG saw one game of action at the 435fps power level using a Prometheus M135 spring. Giving up full-auto for 35 more fps wasn't worth it in my opinion. I cracked the gearbox to see how the internals looked.
Not much wear, which was to be expected. Note how the brass trigger contact has been removed from the back of the trigger, this is how I removed full-auto. It's easy, and reversible.
I measured the spring and noted it had lost almost a centimeter of its original length, which explains why it dropped from the original output of 455 to 435 fps in less than 1000 rounds. All springs settle, but for a $20 spring, this is lame.
Looking at the stock Marui piston showed some wear on the pickup tooth, and the metal release tooth.
However I'm confident that a TM piston can handle a 400fps build very reliably. But, for 475, I wanted peace of mind. Enter the ZCI steel rack piston.
Because I wasn't worried about weight (more on that later), the full steel rack should last the life of the gun.The rack slides off, for extreme builds, it's recommended to epoxy this in. Note how the second to last tooth is missing. This is critical for proper AOE (more on that below).
Onto the cylinder head, the stock TM piece showed absolutely no signs of fatigue and didn't need to be replaced. But I picked up a ZCI G36/AUG cylinder head for $6. And it was shiny and red. So in it went.
The rubber pad came off easy. Too easy, actually.
But I was replacing this with Scatterplot sorbothane and a neoprene anyway, so that made that part of the installation all the much easier.
This will serve two purposes, both have to do with reliability. First, it will cushion the shock of the heavy piston assembly slamming into the front of the gearbox. And second, it will correct the AOE, or angle of engagement, with the sector gear and the pickup tooth of the piston. Here's is the final angle:
Another note on the ZCI cylinder head. It has a small tab in the front that is designed to fill the space direction above the nozzle. If this functions to more firmly mount the cylinder head in the gearbox, or just transfer more shock to the gearbox, I'm not entirely sure. But all V.3 gearboxes are not the same, and yours may not have this cutout.
All of this was done to increase the reliability and durability of the gearbox in order to better handle the power increase. That came in the form of an M140 and M150 spring from Core. I wasn't entirely sure what the final output would be so I bought multiple springs to try and get as close to the 475fps maximum as possible. And at $6 a piece from Clandestine Airsoft, hard to go wrong.
Decided to go with the M150 first. Knew this would be a pain in the a$$ to get it so my masterful technique is to desperately hold the cylinder down against the table with my left hand and mangle the spring guide with my right until it miraculously set in. Grabbing the other side of the gearbox shell while still holding the cylinder with my left and then performing some serious handrobatics to get it closed. Be careful while doing this. Make sure no one or no thing that is easily broken is nearby, because if you lose this the spring guide can do some damage.
Once closed and put back together. I tested the gearbox alone to verify that it would cycle.
It did not.
It locked up almost immediately. With a 25-50C 11.1v lipo, I knew it wasn't a current problem. Looking at the sector gear confirmed my suspicions.
With the added thickness of the spring, it had compressed fully before the sector gear could complete its cycle. Note how the pickup tooth of the sector gear is just barely peeking out from the gearbox?
So I carefully started to crack open the gearbox, slowly, carefully, which is when it popped open flinging the spring guide on the floor about six feet behind me.
I removed the delrin spacer from the spring guide and installed the M140 spring to be safe.
Putting it all back together and verifying that it would cycle this time. I plugged it back into the AUG and chronoed a respectable and stable 450-451fps. Great, but not my goal. Taking everything apart and reinstalling the M150 spring but without the spacer this time, it cycled great and chronoed 474-477fps.
A note on piston assembly weight: While there are varying theories as to why this is, a heavier piston weight seems to increase the final power output of longer barrel guns with heavier BBs. Granted, a heavier piston assembly slows down cycle rate, this is irrelevant in a semi-auto DMR build. My piston assembly is the ZCI full steel rack piston mated to a PDI POM piston head with the bearings. Losing the delrin spacer on the spring guide doesn't matter as the bearing on the piston head wasn't a problem as the common wisdom is a bearing on one end of the spring is enough.
The approach for a high-speed, short barrel weapon is nearly the opposite. Check out my G36C build here for more info on that: viewtopic.php?f=18&t=40513
in summary, great success. More to come with an explanation of my R-hop setup.
