See the effects of an M120-130 on Nylon Gearbox

ACR J10 stock gearbox shell.
18:1 SHS gears
Chi-hai blue motor
Fighting Bro m120 spring
33 cm bruisemaster barrel
Yellow arrow cylinder head has pushed forward into the pin
Red Arrows ridges have formed from the force of the cylinder head smashing against it

This allowed the bottom of cylinder head to move forward putting nozzle on an angle

SLR stock gearbox shell
12:1 SHS gears
SHS high torque motor
SHS alloy piston
SHS M130 spring
Stock barrel cut to 22cm
piston being pushed upward at pickup has put an angle on the rail which allowed the piston to twist upwards and not be picked up properly by the sector gear anymore.
Pretty badly beat up and the pickup tooth of this SHS alloy piston is a little lower than the Blue nylon SHS piston. I also think the SLR gear box mag feed switch area has made a weak point on this G box allowing it to flex more there as well.


Thanks for documenting @Kamchop. How many hours of operation or how old is the build?

Hey @Kamchop did you short stroke your build to run the 12:1 gears. I went trough a couple of smashed pistons before @Rattler showed me how to short stroke my 13:1 set up. I am running a metal box so the weak point was the piston and piston head and not the nylon piston guide in the box.

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I have short stroked all my builds by 2 teeth. Have a 12:1 nylon box M130, 13:1 MK 92 box will have M145 / M155 in it, and 1 18:1 nylon box M120
and 1 18:1 M135 Retro arms box.

  1. To run longer AS springs with out over compressing.
  2. Slight faster cycle times
  3. See stuff on Youtube and wanna try it out lol
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Yeah the loss of 2 canines seemed to be the magical number for my 13:1 build.
Are you trying to keep your box’s running around the 300 FPS limit for games?

plan is too run big springs and remove teeth to get under 300fps eventually. Then it becomes a game of lightening the piston running pom piston heads to make sure piston is fast enough for the quicker cycle times.
Just like how responsive this setup is for semi auto
ATM the ACR with 33cm bruisemaster FB M120 does 400fps and the SLR with 22cm stock alloy barrel does 320fps
To fix the J9 / 10 nylon box i will masking tape the cylinder head and use JB weld to rebuild front portion so can keep running to next failure.
I use these 2 builds to test Gels so have no idea round / gel count BigWeetBix

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So you must still be running standard gears and not short stroked on the ACR to get 400FPS.

That is my dilemma now. I’m in uncharted waters. I use to try to max out FPS and now I’m not allowed to play cowboys and Indians because they capped it at 300 FPS. So I have like you put my efforts into getting higher ROF rates.
You can buy brand new Gen9 nylon box casing for about $18 bucks.

ACR is short stroked 2 teeth. The 7.1mm bruismaster adds about 40-50fps over a 7.5mm barrel.
Have 3 spare Gen9 nylon boxes wanna see how long this one lasts under abuse.
All four SHS gear sets in my builds have 2 teeth removed and all 4 SHS 14 teeth metal racked pistons have 2 teeth removed at release and depending on AOE may have half a tooth removed as well on pickup side.
Video of it in action

Hopefully can view.

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Your FPS is high probably higher than you want it at the sacrifice of only getting 24 RPS. I’m trying to get my FPS down to 300 and ROF over 30.
This is my figures just with a nearly stock Azrael MPX except for the 13:1 gears and better t-piece seals as supplied by @Rattler Custom fabrications inc ltd

Those are some damn fine numbers.


When i put in cycle time for short stroke was for semi auto response not ROF. I want this build at 400fps as a test build, its not for field use.
When this build had a 1.1mm stock spring was 260fps. following pic is for laughs. ROF at 60 lol

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Update on the SLR build. Ended up adding a sorbo pad to move piston back a little from worn rails. Worked fine for another 5 mags about 1500gels. Then lost fps. Opened it up and gearbox had finally split at the front from the corners to the bolt holes. Maybe put a MKv2 in it.