Many Japanese designed GBBs are designed for durable performance on HFC134a (aka duster gas). While some Japanese GBBs perform a little on the anaemic side on HFC134a (sissy kick), Western Arms (WA) guns are well optimized shooters on HFC134a. Use of higher pressure gas like propane (the gas formerly known as green gas) results in very strong kick and higher muzzle velocity but risks slide breakage.

WA guns are clearly designed for lower pressure HFC134a which exerts around 85psi at 70F whereas propane exerts approximately 130psi. The increased pressure causes much faster slide recoil which results in higher impact loading when the slide hits the end of the recoil cycle. I have replaced several WA slides cracked or completely broken from propane use.

A restriction can be placed in the flow path of gas supplying the blowback mechanisms in most WA GBBs. WA uses a blowback scheme unique to WA guns (Magna blowback) which strongly separates the stages of projectile launching and slide recoil. Because of this, it is possible to reduce the flow of propane into the blowback internals so less impact is dealt to the slide with each cycle without reducing flow to the breech.

This is where the Flow Restrictor (FR) comes in!

The FR is simply an accurately manufactured brass disc which can be placed into the blowback internals to reduce slide impact. The design of the restrictor is such that blowback performance of an FR upgraded gun is the same as a stock WA gun propelled by HFC134a. Because of the way the Magna blowback scheme works muzzle velocity is not affected. This means that WA users can use economical propane without overstressing their stock plastic slide and benefit from higher muzzle velocity! The gun I purchased to test this product in is a 5” WA Xcellerator which fires a 0.25g bbs at approximately 315fps!

I have proven through testing that propane powered FR upgraded WA GBBs of many types blowback equivalently to stock guns propelled by HFC134a. I have also set up my once new Xcellerator to fire over 20,000 shots to show that my upgrade will durably reduce slide recoil.

This one is a funny one...

I spent a lot of thinking, drawing many schemes for accurately measuring the recoil velocity of a GBB slide. Schemes started with electronic timing equipment (optical switches which trigger timing circuits) and progressed to a mechanical solution. While I was finally starting to set up my first test, an idea struck me when I looked at my GBB clamped in a vise.

Western Arms Slide Velocity Test
requires: Quicktime

If you remember your high school physics you’ll remember that a horizontally thrown object will land a distance from the point of release. If speeds are relatively low, air resistance has very little effect so the horizontal distance travelled will be a result mostly influenced by the speed of the projectile.

This velocity measurement is very simple! I invite you to try it yourself with your GBB to confirm my findings if you pick up a flow restrictor.

Lots of data to absorb here. I repeated the test for several WA guns including:

Results were collected for different upgrades (stock, with FR, high flow valves) and propellants (HFC134a, propane). Other guns were also tested, but they weren’t mine so I couldn’t gather enough data to show trends. Their slide speeds were very similar to the results presented here owing to their similar design.

I also found that FR upgraded WA M9 GBBs recoiled similarly (to HFC134a performance), but they suffered from a mag capacity issue so the FR is not offered for M9 variants yet. M9 variants typically were short by 8 rounds (i.e. insufficient gas to fire the last 8 rounds). I have noted this issue even without a FR upgrade. It’s caused by a difference in expansion between propane and HFC134a. One ml of HFC134a expands to a larger gas volume than with propane. I have confirmed this myself by modifying M9 mags to hold more gas. This was done by trimming the overflow tube which is part of the bottom block of the mag (not by making the reservoir bigger). Two of my M9 mags hold sufficient propane to fire every shot now. I will be outlining my modification procedure soon so M9 users can use a M9 version of my FR (smaller diameter FR).

Sorry for the mix between metric units and SAE units. I think airsofters are more familiar with fps but I’m more familiar with a metric gravitational constant. Airsoft fasteners and gun dimensions are all metric, but raw bar stock materials that I machine stuff in is all SAW. In my world you have to be fluent in both metric and SAE.

Each datapoint represents a shot taken in a series of shots spaced 30s apart (X axis indicates time). The top heavy read line follows the recoil speed of an unmodified 6” SVI propelled by propane. Average recoil speed is around 13.5fps. Below are results for FR upgraded SVIs very close to the recoil performance of an unupgraded gun firing HFC134a.

Recoil speeds for a stock WA gun firing propane are significantly higher than a stock gun firing HFC134a. Nearly 30% higher (29.4%). Propane firing FR upgraded WA guns perform recoil identically to stock WA guns firing HFC134a.

Here, I experiment with various setups with and without high flow valves to see if the FR reduces recoil velocity.

WA certainly changed their design with SCW guns. On HFC134a they recoil nearly as hard as a propane powered non SCW gun with no FR. I hope WA has strengthened their slide design or I’ll have to release a SCW specific FR. The effect of high flow valves on recoil speed is not very high. Just 7% faster on average.

Although the recoil speed test can indicate if a propane propelled FR upgraded slide has similar recoil behaviour to a HFC134a propelled unupgraded slide, it does nothing to assess the durability of the rest of the GBB.

To get an idea of the wear issues associated to firing propane I built a lifecycle tester to repeatedly fire a WA 5” Xcellerator to get an idea of the long term effects of propane use.

The lifecycle apparatus is pretty simple. It consists of:

Life cycle testing apparatus

Life cycle testing apparatus in action

When the crank arm rotates, the linkage is pulled which pulls the trigger in turn. Recoil cycles are counted by the solenoid counter when the slide fully recoils and hits the arm mounted into the microswitch. The magazine is tapped to accept a remote line fed by a disposable 465g Coleman tank. I put the tank in a bucket of warm water to assure that repeated firing does not chill the tank (resultant pressure reduction). WARNING: Airsoft Innovations Inc. does not recommend intentionally heating propane containers. A pressure gauge was put in on a “T” connection for to monitor pressure. Approximately every 1,000 cycles, the test was stopped for a quick inspection to assess wear.

Propane exerts higher pressure than WA GBBs are designed for. To assure reliable hammer strikes, a Guarder hammer spring kit was installed. Stock hammer springs can sometimes cause light strikes which do not actuate the magazine pneumatics properly.

The following failures occurred during testing:

In both SCW and non SCW GBBs, propane propelled slide recoil speeds of FR upgraded GBBs is quite similar to HFC134a performance with an unupgraded slide. Slide recoil speed is significantly higher with propane and no FR. Although the feel of such a fast recoil is certainly nice, it pushes the margin of what stock plastic slides are able to handle.

Muzzle velocity is significantly higher with propane use than HFC134a (85fps faster!).

Stock hammers appear to be pretty durable, but not infinitely so. 12,500 cycles amounts to quite a lot of shooting (5 whole bottles of Guarder BBs) so this may not be an issue. This experiment has been executed only once. A single hammer failure only anecdotal. One cannot conclude that stock hammers are uniformly durable to 12,500 cycles. However, the failure does indicate that stock hammers can fail when driven by upgrade hammer springs. In my airsofting history I know of 2 hammer failures in WA semi auto GBBs including this one. Given the fact that I have seen dozens of WA GBBs now (with only 2 hammer failures) I think hammer failure is not a very significant issue.

The tearing magazine seal issue is annoying. I have found little lifecycle difference between stock WA mag seals and Guarder replacements. Luckily they are pretty cheap ($3USD) and take seconds to replace. I’m working on an everlasting mag seal to deal with this issue. So far, things look good. I’ve made two prototypes which have lasted well over 3,000 shots with no sign of deterioration. I’m currently dealing with manufacturing issues.

The wear issues of using propane in a FR modified WA GBB are probably the same as using a metal slide upgrade. An aftermarket metal slide does nothing to protect the hammer or mag seals so a metal slide upgraded gun would be subject to the same failures. I have noted mag seal tearing with my metal slide upgraded SVIs. Unfortunately I have very little lifecycle data on the use of HFC134a. I have a bad habit of upgrading things and blatting away with propane immediately. Once you’ve appreciated the performance of propane, it’s hard to go back.

85fps is a big jump in muzzle velocity which is inexpensive to attain. With just $33 of upgrade parts (AI FR, Guarder hammer spring kit, 2 spare mag seals) you can have a durable and powerful GBB.