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Rating: 7 votes, 5.00 average.

Semi only? Yeah, I can do that.

Posted 01-26-2012 at 11:15 AM by Rygar
I have this DMR you see, and in order to make it legal to field, it has to be semi auto only. I got an incredible deal on a Mosfet that is active braking only, so I needed to mod the gun itself.

Since I really didn't have a clue as to how this was done, I started searching. I had asked a couple folks who might know how to do so, and I got a few suggestions. Most of them involved some major cutting on the Selector plate though.

At this point, I should add, the DMR had a little hiccup when it came to shooting on Semi auto normally. I would pull the trigger and sometimes get two, maybe even a three round burst before it would stop. This should have been something that I was concerned about, but my mind was stuck, one tracked, into making it semi only. I guess I figured that once I found the right mod, the issue would correct itself. I was wrong.

In the research I did, I found a very quick and easy mod to the selector plate that would make it into what I needed, and easy to turn the gun back into one that is Full Auto capable later if desired. The only thing I needed was an extra selector plate. One with the mod, one normal. the only drawback to this is you need to completely disassemble the gear box in order to swap the selector plates. Beh, take the good with the bad, right?

So, onto my story. The evening I had decided to do this, I ran into a problem that would make me learn more about the trigger assembly and how it functions in a Version 2 gearbox than I had before. I knew the basic function, but never really delved into just how and why it was built the way it was.

I started with my spare selector plate. The gun is an A&K SR-25 for the DMR. For the most part, it is a Version 2 gearbox, but the cylinder and piston is longer in the front. I got it off of another AO member who had started it as a DMR build already. Several upgraded parts, including the gear set.

For those of you who are new to the world of Gearboxes, this is the selector plate. It changes the guns fire sequence from safe, to semi auto to full auto at the flick of the switch (which moves this plate)

I had a spare gearbox that I was using to see what moved when this was moved, etc. learning the why's and whatfor's of this part of the GB. I noticed, that the little part at the top had a spring that attached at one end to the selector plate, but attached the other end to the cut off lever that is inside of the GB. Until this moment, I honestly didn't know or care what the cut off lever was for. Today it jumped out and screamed at me "LOOK AT ME!! YOU WILL HAVE TO DEAL WITH ME TODAY!!

When the selector plate is moved to the semi only position, the end of the cut off lever attached to the spring does not move. when the plate is moved fully to the rear when Full Auto is chosen, the end of the cut off lever is moved by the selector plate and held in position. Interesting. This explained why the mod I found was done in the way it was. If the cut off lever is not moved, no matter where the selector plate is positioned, then the gun will only fire in the semi auto mode! AWESOME! I set to work, confident this was going to solve my problems and make my gun field legal!

I brought out the dremel too and put on the cutting attachment. I simply and quickly elongated the area where the spring and cut off lever were. I cut out a small notch, about 1/8 inch total and cleaned up the cut of burrs and such.

I did have to stretch that little spring just a tad. When the S. Plate was moved back into the full auto area, the spring was just a little short and would pop out. Reason, the cut off lever on the other end didn't move up anymore keeping the spring under compression. I accidentally stretched mine a little too much, but it still worked. After installing the modded S. Plate, I set back to putting the GB together again.

It's at this point I need to mention how much I HATE anti reversal latches! Evil little things!!

Box together and back into the lower receiver. Grip and motor on, plug in the battery for a test and.......utter disapointment.

It didn't work. I would pull the trigger and get more two and three shot bursts and the occasional single. This was NOT acceptable!

Now I had to get down and dirty. I needed to closely examine the whole process that happened inside the GB. Exactly WHAT made the gun shot single shot, and how it worked.
To cut down on another three paragraphs of my babbling, I discovered that the cut off lever engaged the little contact sled that is pushed forward by the trigger itself. But what told the cut off lever to move and hit the contact sled moving it back off the firing contacts? Hrmmmm.

The sector gear! I never really noticed it before, but there is a nub on BOTH sides of the sector gear! One moves the tappet plate for the air nozzle, the other on the underside engages the cut off lever for the Semi Auto setting. I was getting excited now! I put the sector gear in the box and spun it around while activating the trigger for a semi auto shot. I found the gear would spin two or three times before it would hit the cut off lever enough to make the contact sled reset.

Bringing out the trusty calipers, I gave the nub a measurement. The new gear measured 7.17 MM. I then brought out the stock gear and it gave me a 7.45 MM. NOW we're onto something here. I tested the stock gear and it activated that cut off lever each and every time! Who would have thought that 3 tenths of a MM would make such a difference!!?

To condense a little more, I swapped the tappet plate nub on the new gear and put it on the stock gear, and since they were the same ratio, put in that stock gear, and got the box back together. A quick install of the grip and motor for a test gave me a grin from ear to ear. It was fully semi auto only now!!! WHOOT!!

Took the motor and grip back off, finished putting the gun back together and gave it another test. YUP! It is now a fully field legal DMR. Full auto has been disabled, but yet, it won't be a complete PITA to switch out the modded selector plate with the old one to bring FA back to life.

Hopefully, later today, I will be picking up my MOSFET with active braking, get it installed, and then all I'll need to do is sight it in with the BB weight of my choice. I still need to upgrade the piston tube and head, but those are hard to find for the really good ones, and the ones I have found are pricey for my budget. Right now, it fires and chrono's around the 460 - 470 FPS mark. I'm thinking the new piston head, a new nozzle, and a new bucking will increase my air efficiency and boost me up to 500 FPS by the time I'm done. Eventually, I would like to try the R-Hop mod on the barrel, but that's for later and another blog.

I'm actually kind of glad that I had the problems I did the other night. It gave me (okay, forced) the opportunity to learn a little bit more about the version 2 gearbox that I had blissfully ignored until now. I love learning new things and love to *tinker* (tinker gnomes ain't got nuttin on me man!! lol ), and since the overall experience worked with the mod and solving the mystery, I am a pretty happy camper.

I hope this terribly long story of my trek to achieve Semi only fire on my DMR has helped someone along the way. If you have made it this far, then my thanks to you for being either patient enough, or for being a total masochist to read the entire thing! LOL

Catch ya on the field...hopefully in the sights of my new DMR!
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Total Comments 6


  1. Old
    Panda's Avatar
    Did you ever get that free float off?
    Posted 01-30-2012 at 07:52 PM by Panda Panda is offline
  2. Old
    cletus MaxmimusIII's Avatar

    R hop up

    Hey Rygar, this is actually a little ahead of schedule but I am currently putting the finishing touches on a drop in ready pre-tuned R-hop barrel kit, I haven't really mentioned it much on AO yet but if anyone's reading this the R-hop is very effect I can get about 300ft at 400fps of course heavy bbs are a must.
    Posted 01-31-2012 at 01:28 PM by cletus MaxmimusIII cletus MaxmimusIII is offline
  3. Old
    Rygar's Avatar
    @ Panda, No. Never got it off. Made a special trip up to Airsoftsmith, and even they couldn't get it to budge. I'm gonna try soaking it in some penetrating oil for a while and get a pipe wrench on it. Hopefully that will work. Until then, it will shoot about a half a mag before the jams start. I'll live with that for now.

    @ Cletus. Swervin was telling me about that at our last Open play. watched the youtube video and visited the site he linked me too. Looks awesome..but, I just don't feel comfortable giving it a try myself yet. Now, I already have a good barrel, if your rate is reasonable, I would be willing to have you do an R hop for me on my barrel. shoot me a PM and tell me some more.
    Posted 01-31-2012 at 05:35 PM by Rygar Rygar is offline
  4. Old
    If you need a replacement spring for the cutoff lever. The flint spring from a bic lighter is the right diameter just cut to length.
    Posted 02-01-2012 at 12:12 AM by swervin45601 swervin45601 is offline
  5. Old
    Jonezy's Avatar

    Send me a PM when you get the chance. I have a gift for you.


    Posted 02-01-2012 at 09:29 AM by Jonezy Jonezy is offline
  6. Old
    hotballz87's Avatar
    just a quick help on when you get your mosfet, Most mosfets with active breaking make the anti reversal switch obsolete, so you dont have to fight with it anymore,
    Posted 02-02-2012 at 09:00 AM by hotballz87 hotballz87 is offline

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