Airsoft Canada - View Single Post - WETTI/AWSS - M4/SCAR/HK416

m102404 · June 13th, 2011, 14:50

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.

June 13th, 2011, 14:50	#839
m102404 Tys Trader rating Join Date: Apr 2006 Location: Toronto	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.