M18 57mm Recoilless Rifle in the Fiftieth State

By Mark Genovese

The first time I saw one of these was on an old Korean War black and white newsreel, years ago when I was a kid back in the Midwest. At the time I was simply amazed, this was right out of a Flash Gordon episode. To us, this was as “Star Wars” as it got. In all the years since then, I’ve only met one grizzled front line Korean War combat veteran who actually used this beast in anger, and let me tell you he really hated it. “Everytime I pulled the trigger, a four foot flame would belch from the back of the weapon like a neon sight announcing to the Commies to adjust their fire”. But for me, it would remain one of the coolest firearms ever.

While cruising the show tables at the 1997 Soldier of Fortune Convention in lost wages, Nevada, I spotted my boyhood fantasy firearm perched eye level on a 1917A1 tripod, with a for sale sign on it. Not having $4500.00 in cash on me would put this project off the main burner for the time being. Several months later while paging through Military Vehicles magazine of Morristown, NJ, my eye caught an ad by Hayes Otoupalic of Missoula, MT, who is a leading museum consultant specializing in U.S. Militaria from 1833 to 1946. He was offering for sale the best selection of recoilless rifles I had ever seen in one place. He has the M18 57mm, the M20 75mm and the M40A1 106mm – what a monster that was. They were available as unserviceable according to BATF specifications, remanufactured as non-firing weapons ready for display or sub-calibered to the .50 BMG.

Now this really started to sound interesting to me, especially since I was in the process of liquidating some of my pre ban collection to pay down on the plastic. I had yet to purchase a .50 BMG of any type to test and evaluate for SAR. I gave Hayes a call and talked story about the gun. It appears he imported them from Italy as deactivated parts but very complete with its M86F telescope and case, front extended handle, monopod, complete bipod assembly, overall cover and bore brush. Originally manufactured by Firestone Corp. (that’s right, the tire people) for the U.S. Ordnance Department.

The one that I own was remanufactured by Black Mountain of Florence, MT for Hayes Otoupalic. They did a fantastic job. I’d dare ya to try and show me where they rewelded the deactivated cut receiver, but it was all done to ATF regulations. The subcaliber device is quite simple yet effective. They take a .50 BMG M2 heavy barrel with two stainless steel washers, one with the outside diameter the same as the inside diameter of the M18 barrel and the other with the inside diameter of the breach. The barrel is then headspaced and both washers are permanently welded in place. There are widely differing opinions on the safety of this style of subcaliber conversion I’m afraid.

The U.S. Military originally trained with a very different subcaliber device, essentially made of a 57mm cartridge case, an inert projectile, a barrel bushing, a machine gun barrel and a lock nut. Simply put, the projectile is drilled through the center to receive a standard A5 .30 caliber barrel, then the subcaliber device is inserted into the M18 chamber using normal loading procedures. The safety issue is the difference in chamber pressure between the .30 caliber and the .50 BMG. The .50 BMG has about the same kinetic energy as an exploding hand grenade. The M18 was designed to vent the violently expanding gas from the 57mm round through the rear breechblock. The escape of these gases is controlled so that no movement of the weapon occurs. This is the recoilless principle.

As a .50 BMG rifle these gases are now contained within the receiver and the M18 breechblock is now acting as the locking bolt. If one takes a close look at exactly how much material is left between the breech block vent holes, one comes face to face with the academic questions, should I stand behind this thing while it is fired? I myself have put over sixty of the Greek manufactured and marked “HXP 74” rounds through this firearm without a hitch. I later examined the breechblock with a magnifying glass and found nothing unusual. That’s not to say something couldn’t happen in the future not withstanding our litigating lifestyle. But in my opinion, it appears to work just fine.

Sighting in this firearm can be a bit of a challenge. The 1917A1 tripod is a given, as many sandbags as you can find and one extra helper wouldn’t hurt. For the target we used a 36” x 12” x 1 1/2” steel plate designed as a counterweight in Otis elevators and donated by them. It was spray painted white so we could see it on the 225 yard berm in Ukumehame firing range. The best way we found to first get in the ballpark was to kneel behind the gun and bore sight straight through the open breech, then bring your M86F telescope traverse and elevation on target. Then load a round, cock the firing assembly and squeeze the handle grip trigger. Even with its muzzle ported for recoil, be prepared. This puppy still likes to come right off the ground, even with the sandbags. After several rounds we heard a loud satisfying gong and watched that sixty pound piece of steel spin like a top.

We spent the rest of the day dinging that steel. I did notice the firearm has a very tight headspace. Closing the rotary breech takes some effort, because of the subcaliber device the original extractor is omitted from the firearm. The first time you try to remove the spent .50 BMG shell you will wish you had fingernails of steel, but most people don’t. What worked best for me was a car tire iron spoon that I painted black to match the weapon.

To complete this project, I had Master Carpenter, Mr. Tom Johnson of Kula, Maui, build an original looking solid wood transit chest, including casters and rope handles. Last but not least, a set of custom oilboard stencils for the chest from Rick Larsen of Westbury, NY. Aloha from the Valley Isle.