BMG M1919A4 Gas Gun Conversion

Here are a few notes of my take on making a "gas gun" or machine gun simulator. Here is how am making a combustion chamber.

These combustion chambers are all based on a direct fit to a Browning M1919A4 barrel with no change to the barrel. I bought a water damaged barrel (~$17 sarcoinc.com) to fire blanks and to use for a gas gun. This will save wear on the barrel that I use for live rounds. Also it uses an actual barrel for an authentic look.

I plan to use a gas gun simulator kit from Steve Smith.
     steveleesmith AT hotmail DOT com

See WW2 Steel - Gas Guns

Notes

All the letters refernce the drawings that follow the text in each section.

All dimensions are in Inches unless anotated.

Gas Flare Cap Method

This is very similar to "Welding Method" below. Major difference is the use a a flare cap vs. the 24MM nut. The flare cap is the correct thread.

A Google search for "15/16-16" showed that 15/16 Gas Flare fitting will work and a trip to the hardware store proved it. I was able to buy a Flare Cap in Brass that fit perfectly on the breech threads of the M1919A4 Barrel. Check the drawings if you are using this for another gun.

This all could be made using a pipe coupling with reducing bushings.

The parts are fitted via sanding and dry tested until everything is satisfactory. The parts are clamped in place and tack welded to ensure an accurate alignment. Care must be taken when final welds are made to not warp the alignment.

Thread one end of the 1/8 pipe to 7/16-20 for about 3/4", long enough for the 2 washers, nut and the flare cap (B, E, G and H).

The use of a steel pipe inside of the barrel's chamber could cause chafing in the barrel. Taper/thin the end of the 1/8" pipe to fit a section of an expended catridge case (J). This forms a brass barrel chamber protector or bushing and will help to seal the chamber to force the burning gasses out of the muzzle and not back out of the breech..

Drill a close fit hole for the 1/8" pipe in the 5/16 washer.

Drill and tap 7/16-20 threads in the center of the Flare Cap (H). The use of 7/16-20 is used to avoid brazing as the as is brass. I just prefer not to braze. You can eliminate some of this, if you braze the cap to the 1/2 washer (B) and elimiate E and G.

Weld the steel parts (A, B, C, B, E and G), drill and tap the jet holes and screw the cap and 1/8 pipe to the combution chamber. Note, the 7/16 nut might need to be welded to the 1/2 washer (B) as it will not fit through the opening in the 14MM nut ... check assembly dry and don't end up having to grind a weld out or otherwise cut the project up, because of a misplaced step.

A M14-1.25 Nut to fit Spark Plug (might weld on inside)
B 1/2" Washer, relief for A
C 1" Pipe 1.5" long
D 1/8-27 NPT for Jets (x2)
E 7/16-20 Nut on end of F
F 1/8 Pipe, taper to fit chamber, may fit with an expended case "bushing"
     Thread end 3/8-24 x 3/4
G 5/16 Washer, drill for close fit to outside of F      Combine B ang G using a fender washer
H 15/16-16 Gas Flare Cap (Brass)
J Brass Cartridge Case End, Bushing Over 1/8" Pipe
K Brass plug, drilled #51 (Orifice)
L Brass Elbow - 1/8 Male Compression to 1/8 MPT

Weld A, B, C, B, E and G
All units in Inches
Loose Tolerance
Not to Scale
Mild Steel (e.g. A36 etc.) no special heat treatment

A M14-1.25 Flange Nut, Steel
     (Tap a M14-1.5 nut to M14-1.25)
     Mounted the nut inside to reduce the length.
The rest of the parts are the same as above.

Jets must be offset

a) To Mix Gases

b) To avoid oxygen from disturbing the fuel gas jet, as the oxygen pressure is much higher than the fuel gas.

The offset can be along the long axis, about 0.1" - 0.2". Or the offset can be made by drilling the pilot holes on a tangent to the inside hole. The tangential holes on opposite sides makes the gases swirl. A $5 word for this is cyclonic.

The jets are made by tapping the ends of the compression/MPT elbows (L) to accecpt a plug (K) that is drilled to about a #51 drill. This orifice is necessary to help prevent the pressure from the combustion from damaging the solenoid valves. Either elbow can be for Fuel or Oxygen.

First Try

"Untested - For Historical Purposes"

I asked a machinist for a quote to make a simple "nut" with the 15/16-16 threads and the R drilled hole for the 1/8" pipe threads. The 1/2" hole and or 14MM threads could be added later aligning on the first hole. If a 14MM nut is used for the spark plug, the chamber must be in a weldable steel. Otherwise you can use Aluminum or Brass of other soft, easy to use material. It has been about a month since I sent a drawing and asked for a price and I have not heard back from the machinist.

This method is combined with the Machined method to form the chamber, spark plug and jet threads. E.g Drill the hole and tap them ....

1.25" Square Stock or Hex
Must fit inside of M1919A4 Receiver
Mild Steel (e.g. A36 etc.) no special heat treatment

Welded Method

"Untested - For Historical Purposes"

This method is the way that I started out since the 15/16-16 thread is non-standard and probably could not be ordered. A 24MM-1.5 metric nut is very close and threads on about 1/2 way enough for the pressures this chamber will meet.

The parts are fitted via sanding and dry tested until everything is satisfactory. The barrel is covered with tape and heavy cloth to prevent welding spatter from messing it up. The parts are clamped in place and tack welded to ensure an accurate alignment. Care must be taken when final welds are made to not warp the alignment.

A M14-1.25 Nut to fit Spark Plug
B 1/2" Washer, relieve for A
C 1" Pipe 1.5" long
D 1/2-20 13/16 Lug Nut
E 1/8 Pipe, taper to fit chamber, may fit with an expended case "bushing"
F 24MM-1.5 Nut, best fit for Barrel 15/16-16 as I could not find that nut local
G Barrel
J 1/8-27 NPT for Jets

Weld A, B, C, D, E and F
All units in Inches
Loose Tolerance
Not to Scale
Mild Steel (e.g. A36 etc.) no special heat treatment

Machine the Chamber

"Untested - For Historical Purposes"

I asked a machinist for a quote to make a simple "nut" with the 15/16-16 threads and the R drilled hole for the 1/8" pipe threads. The 1/2" hole and or 14MM threads could be added later aligning on the first hole. If a 14MM nut is used for the spark plug, the chamber must be in a weldable steel. Otherwise you can use Aluminum or Brass of other soft, easy to use material.

A M14-1.25 Nut to fit Spark Plug (Weld to B)
B Combustion Chamber
C 1/8-27 NPT (3x)
D 1/8 Pipe, taper to fit chamber, may fit with an expended case "bushing"

All units in Inches
Loose Tolerance
Not to Scale
1.25" Square Stock or Hex
Must fit inside of M1919A4 Receiver
Mild Steel (e.g. A36 etc.) no special heat treatment
Can be made from Aluminum or Brass if you tap the spark plug threads.
Use correct drill for the threads in place of the 0.5" hole.
M14-1.25 TAP 33/64".


Last Updated: 2009.03.09 - - 2000-2009
Comments to: Tom Essary -
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