WETTI/AWSS - M4/SCAR/HK416 - Technical Summary Thread

[m102404]

Same on mine.

It's because the bolt of the PDW doesn't actually ride in the charging handle so there's nothing keeping it up.

[boringegg]

ah...ok...i guess i will file that corner down on the upper receiver at some point...

[N_Force]

that is the project I'm working on since I change my PDW to open chamber, using PDW mag is okay but the M4 mag has the same problem like you have. If you take a look on the WE site "how to change to open bolt", you will see the guy testing fire is first single shots and then auto, seems like the same problem at the end but he didn't keep firing it. I think it is the design difference between PDW may and M4 mag(loose from back and fore when it kick back). I have some idea in my mind just I don't have enough time to try out yet.

Originally Posted by shaharov

Having a small problem with my WE SCAR. It's an open Chamber system (I hate people calling it an open bolt scar). Semi works like a charm which is great as I really only use semi. Full auto seems to be inconsistant. Shots drop after about 20 feet and are slow as shit... but only sometimes. Ie, in full auto 1st shot fine, 2nd drops, 3rd and 4th fine, 5th and 6th drops, 7th is fine 8 and 9 drop, etc. There is no pattern in the firing. I was told it could just be the VSR hop up as they tend not to like full auto anyway, but I've heard other stories of the guns working properly. Like I said.. Semi-auto is fine, even when I pull the trigger as fast as I can but it's only when its in full auto that sometimes the bb's have no velocity behind them. Any help would be appriciated

[shaharov]

Interesting.... My problem is on a scar so it isn't the difference between the stang (m4) mags and the pdw as the scar only uses the stang mags. When I get the pdw I have converted and running I'll have to see if this happens. Keep us informed if u figure anything out

Originally Posted by N_Force

that is the project I'm working on since I change my PDW to open chamber, using PDW mag is okay but the M4 mag has the same problem like you have. If you take a look on the WE site "how to change to open bolt", you will see the guy testing fire is first single shots and then auto, seems like the same problem at the end but he didn't keep firing it. I think it is the design difference between PDW may and M4 mag(loose from back and fore when it kick back). I have some idea in my mind just I don't have enough time to try out yet.

[N_Force]

not 100% sure yet since I spent some time last night, charge your mag a bit more than half (not full charge)and have enough gas to shoot all 30 shots in auto and see.

Originally Posted by shaharov

Interesting.... My problem is on a scar so it isn't the difference between the stang (m4) mags and the pdw as the scar only uses the stang mags. When I get the pdw I have converted and running I'll have to see if this happens. Keep us informed if u figure anything out

[shaharov]

Still bbs are exiting with low velocity.

Originally Posted by N_Force

not 100% sure yet since I spent some time last night, charge your mag a bit more than half (not full charge)and have enough gas to shoot all 30 shots in auto and see.

[Ross]

Does it still happen with the hop up turned all the way off? (Aim up, so the bbs don't roll out.)

[subtletrouble]

Hey I lost the rubber insert for the we m4 stinger hopup anyone have a good idea for a replacement? Other than another kit

[N_Force]

those stupid M4 CQB and PDW both firing pin valve locker broken on me and one need to be replaced within 700 rounds shooting. So fragile.

Since I charge mag not in full, it seems work better. If not, I just fire 3 brust and the next full auto is working fine. I'll keep on testing it once I have the firing pine volve locker replaced.

Originally Posted by shaharov

Still bbs \exiting with low velocity.

[Cushak]

Was having bad feeding issues with my NPAS (V3 M4) (would cycle, BB wouldn't leave barrel/hop-up chamber, once a bunch got built up they'd fire out like a shotgun). With a game tomorrow I had to make the original nozzle assy work, trying different spacers, I figured I'd try a sorbo pad cut smaller, and a bit thinner. Got 300 fps, I post this because while skimming this thread trying to drop my FPS on my stock one I noticed somebody saying they were having trouble dropping the FPS to less than 400.

[shaharov]

Originally Posted by Cushak

Was having bad feeding issues with my NPAS (V3 M4) (would cycle, BB wouldn't leave barrel/hop-up chamber, once a bunch got built up they'd fire out like a shotgun). With a game tomorrow I had to make the original nozzle assy work, trying different spacers, I figured I'd try a sorbo pad cut smaller, and a bit thinner. Got 300 fps, I post this because while skimming this thread trying to drop my FPS on my stock one I noticed somebody saying they were having trouble dropping the FPS to less than 400.

Most times people use an NPAS, however this can be accomplished by increasing the size (not tension) of the valve sping.... Wait just realized that's for an open bolt nozzle. I'll have to look at the brass tube nozzle and let you know. Basically you want the valve in the nozzle to close faster, putting less air in the barrel. A different spring that moves the seal/plate closer to the nozzle is what you'd want. I just can't remember where the spring is located

[Cushak]

Ok so I had the feeding issues no matter what I tried, gun was out for game day. was sad, still haven't used it in a game.

Upgrades: RA Tech 6.03 barrel (~363mm), stinger hop, and ra tech fire pin base and valve locker.

Here's what I all tried: NPAS nozzle, stock nozzle, stock nozzle w/ various spacers, Stock nozzle with Npas adjustable valve (surprisingly fit and sealed nicely), shaved hop-up rubber (used on that had a chunk missing due to the sharp edges around the hop-bearing hole on the brass chamber), basically, no hop-up but still sealed.

The feeding issues are the same as other ones I've read through in this thread, BB's just get fed into the hop-chamber area, but only come out in groups.
So it's:
Fire - nothing
Fire - nothing
Fire -nothing
Fire - nothing
Fire - Shotgun blast!
Repeat, number of BB's is random. Otherwise, could function as an awesome shotgun if predictable.

If my memory servers correct, this problem was less severe (though still occured) with a completely stock nozzle. (Shot way to hot, about 500). As soon as spacers were placed behind the stock nozzle/valve to try and drop it, feeding issues. (BTW, stock nozzle is now on NPAS rod. I've tried using gearbox shims between the nozzle and the smaller plastic spacer to ensure correct fitting, no help.

Problem was happening with BB bastard BB's, Armist and Green Devil Bio BB's.

After reading this thread, two things I believe I can still try, sanding down the inner diameter of part #177, and/or decreasing the diameter of the nozzle mushroom. First, I'm going to try throwing the stock barrel back in. If anyone else has any ideas of what I can try, please let me know. If I do find something that works, I'll definitely post it up so others can know one more thing to look for.

EDIT:
On the upside going through this is a good crash course on how these guns work. Which led me to this conclusion: The theoretical function of this gun is as such:

-Bolt is racked, round loaded, everything is ready to go.
-Trigger pulled, hammer fires, gas is released into the brass tube.
-Gas rushes around the valve in the nozzle and runs into the back of the BB. There is now equal pressure around the valve, so it isn't being forced closed yet.*
-BB is pushed down and out of the barrel, as soon as the BB has exited the barrel we have a significant pressure difference, valve is slammed shut and the nozzle is sealed off.
-Gas pressure than forces the bolt back.
-At a midway point, the bolt trips something in the trigger block, and the hammer is pulled off of the valve on the magazine, releasing gas pressure.
-Momentum of the bolt, continues backwards, pulling the nozzle out far enough for another BB to be chambered
-Recoil spring eventually overcomes bolt momentum and slams it forward, resetting everything for the next shot.

Now here's my question, if this is correct, how is it even possible for the bolt to cycle if the BB stays stuck snuggly in the hop-up unit? That tells me that gas is either getting past, or out somewhere.

*Now if the "windforce" of the gas rushing past the nozzle valve were enough to close it (as opposed to the pressure difference closing it), pressure to the BB could be shut off before it's ready. Perhaps a stronger spring in the nozzle could solve this?

Edit: Second thought: If for that split second where there is a pressure difference between the BB/nozzle valve and nozzle valve/gas source as soon as you pull the trigger, and that is enough for the valve to be closed, than really, no pressure would even be getting to the BB's, so it'd just be cycling and loading them up till the gas was able to squeak by the valve before it was shut. Correct or incorrect?

Edit: BB's roll easily through upgraded barrel, so I don't think it's that.

[Stealthee]

I think it's the RA-TECH inner barrel. I had the same problem as you, but when I reverted back to the stock inner barrel (NPAS still installed) it worked fine. I suggest you just stick with the stock inner barrel and upgrade to open chamber barrels when you upgrade to open chamber.

[m102404]

PDW? SCAR? M4?

Basically your gun is short cycling. Either from the valve being closed off too much...or the nozzle not being able to chamber the BB enough.

I have found that the stock nozzle consistently provides the best fit with the stock rubber insert (that the nozzle inserts into just in front of the barrel).

I've found the fit of the nozzle tip with that rubber insert to be very picky...if it's at all sticky and holds the nozzle when it's extracting...then it'll tend to use up a lot of gas or jam the cycle.

A loose/zero stick fit is ideal. Obviously too loose and you'll get gas blow back/by.

Re. the jamming...there's two reasons.
1. The hopup is on too much and is physically preventing the BB's from exiting.
2. The nozzle isn't being pushed far enough forward into battery.

Re. 1....that's usually from modding the nub/spacers/ball bearing and forcing too hard a spacer too far into the barrel. We did that on one setup this weekend and tore the hopup rubber. It's a fine line between too much hop and not enough....I've found using 0.30's to result in the best balance.

Re. 2...the "headspace" is completely a factor of the length of the spacer behind the nozzle. Try reverting it back to stock length. Too short and the nozzle won't be forced far enough forward to chamber a round pas the rubber insert just before the chamber of the barrel. Too long and the bolt might not return far enough forward to actuate properly. For example the PDW plastic spacer is about 1.5mm+the metal washer (dunno why that's there)....I can't remember off hand what the M4/SCAR spacer length is (I wan't to say 4.5mm but can't recall exactly). There is a bit of slack...so I think the spacer can be LONGER by a 1mm or so before it causes problems.

Also..if the tightbore chamber cut is a little too tight...it'll cause the rubber insert to be squeezed down too much and that may block things up. Revert back to the stock barrel. If that works, you'd need to review the I.D. of the chamber cut between the two barrels...and it's pretty fiddling to measure.

In general I'd recommend the stock nozzle body, it's brass base and the NPAS float valve and it's spring. When seeing if the system actually works...shoot it with the valve all the way back (most powerful). Then choke it up bit by bit until you get into the FPS range that you want.

NOTE: that in the winter months, for cool-ish indoor temps, the system is almost fully choked off to shoot 330-ish with 0.20's. Around here many field games have bumped their FPS limits to 450fps since the gas guns tend to perform better when they're allowed to breath a bit.

[Cushak]

Thanks guys, I'll give those things a shot quick. (It's a gen 3 M4, a cansoft one). I know it's not the hop-up, as because the rubber got a rib right by the nub, I trimmed the nub off the rubber ring, and just rotated it out of the way, so all it's really doing right now is maintaining an airseal.

Is upgrading to the open bolt a lot better?