DIY: WE M4A1 “Open Bolt” Velocity Reducer

Speaking of WE M4 nozzles…

I found that the RA-Tech NPAS I own has a tendency to shift itself during firing, causing (up to) some pretty wild muzzle velocities by the end of a day on the field – something I’d rather not subject other players on the field to, provided I can even hit anything with the constantly fluctuating hop up setting required due to increased FPS.


Using the same concept of adding a shim behind the floating valve as per some of the FPS reduction kits for various GBB models, I’ve done this simple little trick to my WE M4 “Open Bolt” nozzles for a couple of years now.

This do-it-yourself mod is as simple as installing a washer or two, allowing for field-legal muzzle velocities while running higher pressure gases (i.e. MAPP) and tightbore barrels (in this case, an Angry Gun 6.03 x 360mm). I found that experimenting with different sizes of washers yielded different muzzle velocity results; thicker washers reduced muzzle velocity to a greater degree (similar to concept of adjustable valve length). I’ve seen other adjustable systems for other platforms that constrict or open up the front end of the nozzle, but I’m not yet sure how stable these are if applied to the WE system, in comparison.

Metal washers seemed to work more reliably over longer periods of time, though were often thinner in thickness. Washers with an outer diameter that were too small tended to dislodge during firing and get stuck in awkward positions within the loading nozzle, whereas I actually never experimented with washers with differing inner diameters but figure this to be an interesting experiment to see if there is any change for trial sometime in the future.

This is something both “Jester” and I did some experimenting with on our GBBR platforms when everything was up and running as good as we could get them to be back then (KJ M4A1 and WE M4A1 Open Chamber, respectively) – same method to the madness of reducing muzzle velocity to fieldable chrono limits, though I could see this being used in other GBB platforms as well, provided that a correctly sized washer can be found and installed.

Remember, gas expansion paired with heavy weight BB’s means a GBBR can pack one heck of a punch when compared to an AEG running the same FPS, when chrono’ed on a 0.20g BB. My WE will easily reach 470 FPS on a 0.20g BB and Green Gas at up to 25 degrees Celsius – often starting at a max of 490 FPS, then 450 FPS after a couple of shots and cool down starts to kick in. Note that 450 FPS is the semi-auto only muzzle velocity limit (400 FPS and below you can more or less do whatever you want until you get real close), but there’s no rule against firing a few test shots before shooting through the chrono around this area. Basically, this is my way of trying not to be a dick to other players on the field, because I know that GBBR + high flow + tightbore + carbine barrel or longer + heavy BB = Joule creep… which really hurts to get hit by – I’ve seen the bleeders my WE’s full output can give, even at 100+ ft.

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