A Discourse on the Function, Hazards and Merits of Liquefied Fuel Gas and Oxygen Powered Machinegun Simulators
By: Marc M, 6th Fallschirmjaeger Regiment
For us the only alternative was to build a "Gas Gun" out of a BATF certified non-gun, in this case a MG34 with a solid aluminum dummy receiver. A non-gun is a simulated weapon that cannot incorporate a functioning receiver and must be designed so that it can not fire a live round. Building a gas gun out of one is not simple, and when completed there are different safety concerns in addition to those associated with a blank-adapted firearm.
The Complete Gas Gun.
This article is not intended to be a guide to building a gas gun or a manual for their use. Rather, it is intended to show the principles behind these weapons, and some of the difficulties encountered in their use, based on the experience gained with the MG34 gas gun fielded by the 6th Fallschirmjaegers.
A gas gun uses a mixture of propane and oxygen ignited by a spark plug, causing a small explosion that simulates the report of a real weapon. In function very similar to a single stroke internal combustion engine, except without the piston. With our MG34, the dummy receiver itself was machined to create a combustion chamber with gas inlet jets and threaded for both a spark plug and the exhaust tube or barrel.
The heart of the gas gun is the electronic circuit board that repeatedly opens and closes two electric solenoid valves that meter the gases into the combustion chamber milliseconds before applying the ignition spark. The regular trigger of the weapon activates a switch wired to the circuit board that starts the firing cycle. The rate of fire may be adjusted by turning a variable resistor on the board. An extremely small motorcycle coil is used to generate the high voltage necessary to operate the spark plug.
On the Fallschirmjaegers' gas gun the circuit board, ignition coil and two solenoid valves are located inside the drum magazine attachment. This is a unique set up as most gas guns have the majority of their electronics in a separate ammo box or container. As we intended to use our MG34 gas gun in the light machine gun role, great lengths were taken to make it a truly portable walk-and-shoot piece. The gun alone weighs only about 23 pounds.
Two miniature quick-disconnect hose fittings, a power connector and an on/off switch are located on the bottom side of the drum magazine. These fittings connect the gas gun via a pair of hoses (with flash-back prevention valves) to the gas tanks secured in a period rucksack. A length of heavy gage two-conductor wire provides power from the 12-volt "gel cell" battery also in the rucksack.
Close-up Showing Magazine Housing.
Both the oxygen and propane tanks use small welding regulators to reduce the tank pressure for control of the gasses to the solenoid valves. We use a miniature aluminum oxygen tank holding 4 cubic feet of gas and a disposable propane cylinder with 16 ounces of liquefied gas. Each gas gun is unique as to what pressures it will function at, however, most run between 5 to 15 psi propane and 15 to 30 psi of oxygen. The loaded rucksack weighs in at just over 18 pounds.
The oxygen tank is probably one of the most dangerous parts of the whole system. As it is pressurized to 2100 psi there is a danger of spontaneous combustion should any flammable oils contaminate the connection between the tank
The Electronics Inside Magazine.
valve and the regulator on the high-pressure side. This could cause a rather nasty explosive tank rupture and although the oxygen itself is not flammable its presence can greatly enhance any existing flame. There is also danger of a propane leak, especially hazardous considering you provide a source of ignition every time the gun is fired. Did I mention that all this stuff is strapped to your back?
There are several reasons a gas gun can malfunction. For ones used in infantry impressions the most common problem is probably the small internal surface area of the propane tanks. Propane is in a cryogenic liquefied state and boils off into gas when in contact with a warmer surface (the wall of the tank) the smaller the tank and the colder the day the less gas is available quickly. This may be a cause of the "goose call" effect after longer bursts of fire, the liquid propane just can't change to gas fast enough and the oxygen just whistles out the barrel. Gas guns seem to be more reliable mounted on vehicles, where weight is not a problem and there is more space to store a larger tank.
Another problem with a pack-pack unit is that liquid propane can slosh around with movement or body position and flow into the regulator freezing it up. Super cold liquid propane can also get into the hoses and flow into the combustion chamber cracking the spark plug insulation.
Our wet, cold, Northwest weather can also give a gas gun problems, excessive moisture can screw up your electronics or cause condensation in the combustion chamber shorting out the spark plug. This can happen even in relatively dry times as one of the by-products of propane /oxygen combustion is water!
Contrary to popular belief, running out of gas has seldom been a problem, one tank of propane can last about two days of moderate firing and a tank of oxygen can last through a day of fairly heavy use. Our oxygen tanks are only about $11 to refill and a new propane tank runs less than $3, a big plus when compared to the cost of blanks a real MG34 can go through in two days of firing.
Propane and Oxygen tanks.
The most common complaints I hear (when the gas gun is working correctly) are: "One guy could never carry that much ammo" or "You never have to change belts or barrels". Valid complaints and I do try to limit myself to short bursts, but it is difficult to restrain others, especially first time users from the "lead finger" effect, it's just too much fun. The gas gun will heat up from sustained fire, just like a real firearm, and if the chamber becomes too hot the gun may misfire. It also usually takes a two-man team to properly operate a portable gas gun as you can't adjust the regulators while it is on your own back. With this in mind and having to deal with freeze ups and spark plug changes, I think you end up with something very comparable to using a real MG.
Another problem with fielding a gas gun in battle reenacting is that it has to be adjusted to work for noise, not muzzle flash. The flame coming out of the barrel is barely visible on a bright day and tends to run more blue than yellow unlike the brilliant flash from Swanson blanks. It can be very annoying, after humping 40 pounds of gun around all day, to watch as target after target strolls through your sights looking around bewildered as they try to figure out where all that shooting is coming from!
To date I have constructed three MG34 gas guns and am presently working on a Browning 1919A6 model, but I am still learning as I go and feel I am far from expert on the subject. I wish to thank all those who have helped me along the way by sharing their experience and ideas in this little known area, especially my fellow NWHA club members. Thanks also to our editor Rob M. for providing me with a starting point and direction for this article and Tony S. for the accompanying photographs.
Editor's Note: The author is a prototype machinist by trade, and is licensed for motion picture pyrotechnic and explosive special effects in the state of Washington where he operates a small part time business specializing in blank firing weapons, pyrotechnic effects and weapons related props for local stage and screen work.
Smith, Steve L. Machine Gun Firing Simulator. Military Vehicles Magazine. January/February 1998.