CASING REVIEW 37mm and 40mm

High Performance Smokeless casings

By GROG 1996-2018 ©3LC Productions


37MM and 40MM High performance casings

Scot provided us with several of his new “high performance” 37mm smokeless casings for testing and evaluation. They come with removable sidewalls, and a new removable high pressure vent hole section. This section has six smaller ventholes that allow the pressure of the lift charge to build before releasing it into the low pressure launch section of the casing, very similar to the military M118 casings, however these ventholes are aimed right at the base of your projectile. They come in any desired length, and sidewall thickness. Scot can customize any of the casing dimensions you desire. These casings were nice in both fit, and finish. The entire casing is machined from 6061 aluminum. The base is solid, and has a powder chamber of .180” deep, and .655” ID. This will hold an adequate lift charge of Bullseye pistol powder to propel a standard half pound projectile a good distance. The vent hole plug area has an OD of .875” (including threads) and is coarse roll threaded for strength. It is cut on top for a hex wrench for easy removal, of .765”. The ventholes are .110 in OD, and there are six arranged around the inside of the plug. It is .610 tall, and the interior is lathed out to .580” ID to a depth of .350”. This provides a thickness of .260” where the vents are. This thickness is adequate to hold the pressure while it builds to a sufficient level to rupture the aluminum used as a wad. I recommend using .016” thick aluminum, cut into a disc shape of at least .815” DIA. The vent plug screws into the base, and the sidewalls also screw onto the base. You can use multiple length sidewalls with the same base. Why? Different payloads. Some payloads work better with long sidewalls, some work better with short. Muzzle blast types and “rocket” type projectiles, you want longer. Flare and some pyro, shorter is better. The sidewalls of these casings were 3.500” tall not including the base. They are .110” thick! This thickness is adequate to launch any type of payload one would want, including (for registered launchers) wooden baton, buckshot, and such. The casing sidewalls fit together very nicely, and are knurled to aid in disassembly after firing. The bases also have a machined series of pin holes .125” in DIA for using a pin wrench to grab and hold the bottom of the casing. This will prevent damage by someone trying to use a jawed wrench to open them.




Above is the removable vent plug for the casings. You use a thin .015” aluminum disc to seal the holes. The vent plug screws into the base interior, atop the powder charge. Having the ventholes spread out, spreads the lift charge around the base of the projectile. The top of the vent plug is machined to allow a socket for tightening and removal. I recommend a small amount of graphite be applied to these threads before their first use, to lubricate them prior to use.



Above you see the base of the new casing. This takes a small pistol primer. I recommend Remington 1 ½ small pistol for this. The threads are visible that the vent hole plug screws into. You simply place your powder charge, place a .015” thick aluminum disc wad on top of the powder, and screw in the vent hole plug. When seating the primer, make sure you seat it fully in the primer pocket. Failure to do so might result in a misfire. Make sure you keep the primer pocket area clean. A dirty primer pocket will result in misfires.


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Above you see two looks into a put together casing. The ventholes spread the lift charge out around the base of the projectile, and provide resistance to allow the high/low pressure system to work properly. These are powerful casings! The sidewalls come in various thicknesses to allow for different payloads. On the left is the thick walled casing, on the right the thin. They also come in different lengths for the same reason. These are the most versatile casings on the market today.



Above you can see the .008” thick copper disc used as a wad in the HP casings for some loads. The left picture shows the disc before being fired in the casing, the right shows one that has been fired. Notice the 6 perfect holes from the lift charge ignition, cut into the disc. These discs are .75” in OD.



Above, you can see a complete base section, with the copper wad under the vent plug, and on top of the powder. When fired, the copper will burst where the ventholes are, and throw your projectile FAR! My normal range was 200 meters with a light projectile and only 5 grains of Bullseye. Not bad at all!


In the above photo, you can see the interior powder chamber area. This area is small, and will hold approx 6 grains of Bullseye Pistol powder. The copper wad sits on top of this powder chamber area (where the rim is above the primer flash hole). I do not recommend using that heavy a lift charge with all payloads. 6 grains will crack most plastic payload cups. I also do not recommend using thicker metal wads. Only use copper, and only use .008 and .010 thick wads! Using anything else might result in injury or death. This casing is powerful, and if used properly, will last a very long time in reloading.






I performed two separate tests with the 40mm thick walled shorter casings, and the 37mm Cap/Cup sold by ASA as projectiles. I prepared the projectiles by loading them with a 1tsp black powder FFFFG burst charge. I sealed the projectile with super glue around the top of the cap/cup. I prepared 4 of these projectiles, and loaded the HP casings with 5 grains of Bullseye Pistol powder, and used a .008” thick copper closing wad to cover the interior of the ventholes. The copper wad prevents the powder from escaping the casing, and allows the high pressure/low pressure chambers to work properly. In normal 40mm casings, the M118 for example, when fired, the burning powder expands and creates pressure until it reaches approx 25,000 PSI, then the copper breaches where the ventholes are located, dumping the 25,000 PSI into the low pressure chamber (the inside of the casing on top of the ventholes and below the projectile). This pressure pushes against the projectile and throws it down range.


The above projectile was used in the tests of the 40mm thick walled HP casings. This is the 37mm fused CUP and cap available from American Specialty Ammo. It is a very simple and reliable projectile that can be loaded with a variety of payloads. For the first tests I used a simple payload of 1 tbs of FFFFG black powder. This provided a safe projectile that supplied a nice smoke burst when the fuse ignited the payload. It also offered a projectile that, if it burst in the launcher barrel, would not damage the barrel. I prepared several of these to test and find a safe load for these. I settled on 5 grains of Bullseye Pistol Powder, using the .008” thick copper wad. This reliably lit the fuse, and threw the projectile around 100 yards. For a light projectile, this is pretty good.


I later duplicated this test using Ninja Smoke mix from Firefox. I videoed this test, and posted the result here:

I decided to do some shot tests with these casings to judge power. I loaded up a short CUP from ASA with 1.8oz of steel shot Gin828 (member here) provided me. The shot is round with a flat belt around the center and ¼” OD. I like the shot because it is much lighter than lead, and that helps in these types of rounds. Lead makes a heavy projectile. Here is a video of the test results:

The recoil on these was not bad, but they did slightly function the carrier in the AR enough to pop the dust cover open. A good sign when you are looking at how powerful a 40mm casing is. The real M576 does the same thing. I used .010” thick copper wad, and 6.1 grains of Bullseye Pistol Powder to load these. The short 37mm CUP from ASA was loaded right on top of the vent plug, filled with steel shot, then a ¼” thick foam wad was placed on top of the CUP, and a cardboard wad was placed on top of that. If stored, I would throw a bit of latex paint on top of the cardboard and allow it to dry.


Using the AR sights, I aimed at the orange target disc. The shot hit approx 12” low and 8” off center. To get the load perfected for use in these casings, additional tests are needed. 6.1 grains is a good starting load for anyone wanting to use these for their own shot loads. Most of the shot penetrated the ½” thick treated backer board. The backer board was not new, only the Tyvek covering was replaced, so I could not tell if some penetrated better than others. I did notice that both CUPs went through the Tyvek where I had weakened the backer by firearms fire.



I figured this would be a good place to show you how to construct your own projectile exactly like that I used in this test project. These are reliable projectiles, and can be loaded with flare composition, smoke, smoke burst, and such payloads. Do NOT use flash powder in these projectiles! There is always a risk, when using HP and other strong casings of cracking the payload cup. If you load flash and this happens, you can easily injure yourself badly. Refer to the safety page on this website to see what can happen.


First you will need a fused CUP and cap as sold by ASA here: ASA Reloading page link If you are using the 40mm thick walled casings like I used in this test, you will not need a bushing. Once you have the cap and CUP, you will need to trim the exposed fuse. Cut both the interior and exterior fuse like the photos below:


Notice the fuse is cut at a 45 degree angle. This exposes the most powder in the powder train for ignition by the lift charge.


Notice the fuse inside the CUP is also cut. This exposes new powder, and makes securing the cap possible.



The CUP is loaded with Ninja Smoke mix from Firefox. Leave a small airspace.



Run a bead of super glue around the top of the CUP.



Place the cap onto the CUP with the A.S.A. on top.



Using a gasket cutter like this, cut a wad out of ¼” thick foam.



Make sure the wad is slightly larger than the interior of your casing.



Allow your payload glue to dry, then place it, fuse down, into your casing.



Here is how it should look, inside your casing.



Take your ¼” thick foam wad and place it on top of your payload. You can add a cardboard wad and some latex paint if you want to store this round for any period of time. Once dry, you are ready to fire!



Here is a simple way to make a buckshot round for the 40mm, using these casings. These are the exact rounds I fired in the above test video. You will need a short 37mm CUP from ASA, a ¼” thick foam wad, a cardboard wad, 6.1 grains of Bullseye Pistol Powder, a Remington 1 ½ small pistol primer, approx 2.8oz of steel shot or similar, a copper disc wad .010” thick and .75” in DIA, and one of Scot Pace’s HP casings. Here’s how they go together:


You will need these casings, the 40mm HP thick walled.



You will need to cut some copper discs out of .010 thick copper sheet. Then, load 6.1 grains of Bullseye, and place a wad on top.




Use a socket in a vise to tighten the vent plug onto the copper wad. Place your shot in the CUP, and insert into the 40mm casing.





Load the foam wad on top of that, then the cardboard wad. Cut these wads slightly larger OD than your casing ID. You want a tight fit! If you want to store these rounds, I recommend sealing the tops with latex paint on top of the cardboard wad, and lacquer the primer. Use blue Loctite to seal the threads on the sidewalls. The rounds will then stay ready to fire for years.


Contact Scot Pace at his website

Contact Chris Reed at his website


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