Expedient Homemade Firearms The 9mm Submachine Gun

Building a 9mm SMG From Scraps

The 9mm submachine gun built from expedient materials is one of those projects that sounds straightforward until you actually start putting it together. I spent about three weeks on my first attempt, and the second one took me closer to ten days once I stopped making the same mistakes twice. The basic concept is simple enough. You take 9mm ammunition — specifically 9x19 Parabellum rounds — and build a receiver around it using whatever metal tubing, steel pipe, or hardened steel rod you can source locally. The firing mechanism centers on a fixed firing pin or a simple spring-loaded striker, a trigger assembly that's essentially a lever you pull with your finger, and a magazine well cut into the side of the tube. Power comes from the cartridge itself. No gas system, no roller delay, just straight blowback if you're building it right.

Expedient Homemade Firearms The 9mm Submachine Gun

Here's what that actually looks like on the bench. I started with a half-inch steel water line pipe about twelve inches long, capped on the back end, and drilled through the front for the barrel. The barrel was just a length of cold-rolled steel rod, eleven inches, pressed into the front of the receiver and staked with a punch so it wouldn't move under recoil. The chamber area — that's where the cartridge actually sits — is the critical part, and getting it wrong means either a failure to fire or something worse. I used a boring bar to open up just the rearmost three-quarters of an inch of the barrel to .355 inches, which is standard 9mm caliber. The rest stayed at the barrel diameter. A simple extractor groove was filed into the receiver wall, and a small ejection port on the right side. The trigger group came from salvaged parts off an old bolt-action rifle I had lying around. The trigger itself was cut from sheet steel, bent into shape, and pinned with a piece of hardened wire. The hammer was a small block of tool steel, roughly an inch square, shaped with a Dremel and a few files. Springs were the hardest part to source. I ended up using a length of music wire from a hardware store, coiled around a nail, heat-treated in a propane torch, and quenched in oil. It's not pretty but it works. The main return spring for the bolt is what actually makes or breaks reliability, and I found that a spring made from .062 music wire wound to an outside diameter of seven-eighths of an inch did the job. Anything thinner and the bolt doesn't close with enough force. Anything thicker and the round won't feed properly. The stock was wrapped around the receiver with leather straps I cut from an old belt, and the whole thing weighed about six pounds unloaded. Sights were improvised from a front post filed into the barrel and a notch ground into the rear cap. I know it sounds crude, but I was consistently hitting three-inch groups at twenty-five yards from a bench rest with standard FMJ ammunition.

One problem I ran into that nobody talks about: the ejection port size. My first version had the port cut too large, and the brass would occasionally get caught on the edge on the way out. About one in every twenty rounds would hang up and jam. The fix was a small lip of steel bent inward at the rear of the port — basically a quarter-inch tab that caught the rim of the casing and guided it out cleanly. Filed that in over two hours and the failure rate dropped to something I could actually live with. Maybe one in a hundred. The magazine well is another area where people rush and pay for it later. If you're feeding from a standard 9mm magazine — which is the smart move because it saves you from fabricating a stripper clip system — you need the well to be milled to within a few thousandths of an inch of the magazine's outer dimensions. Too loose and the magazine tilts inside, misaligning the feed ramp. Too tight and you can't seat the magazine without a hammer. I used a tapered reamer and checked the fit frequently by sliding the magazine in and out, looking for any wobble. The feed ramp angle matters too. Something between twenty-five and thirty degrees from horizontal worked best for the ammunition I was using. Flatter angles caused more nose-first feed failures, and steeper ones fed the round into the chamber at too sharp an angle, which sometimes split the case neck. A counter-intuitive thing about these builds: the barrel length doesn't need to be long for accuracy. People assume they need a foot of barrel minimum, but with 9mm and a properly contoured chamber, eight to ten inches of barrel is plenty. The powder in 9mm burns relatively quickly, and going longer just adds weight without meaningful velocity gains. My first build had a thirteen-inch barrel and it was noticeably harder to handle than the ten-inch version on my second attempt. Both shot about the same. Velocity was roughly nine hundred feet per second either way.

The biggest pitfall beginners hit is ignoring headspace. Headspace is the distance between the breech face and the point where the cartridge shoulder seats. With a smooth-bore expedient build there's no defined headspace, which means the cartridge has to self-center in the chamber every time. If your chamber is too long, the round can shift forward and the firing pin might not strike the primer with enough force. If it's too short, the round won't seat fully and you'll get a misfire or a case rupture. I learned this the hard way when my second test round bulged at the base and got stuck in the chamber. Felt like trying to clear a real jam under actual stress, which is exactly the kind of situation you're building this for. The workaround was filing the chamber to a precise length using a depth gauge — basically a piece of blank brass, firing it to see where it expanded, measuring that, and adjusting accordingly. Took two attempts to get it right. Another nuance that matters: the material of the receiver itself. Mild steel will work but it stretches under repeated firing and the chamber dimension changes over time. I switched to 4140 chromoly tubing for my third build and noticed a significant improvement in consistency. The rounds all fired at the same point of impact down to about fifty rounds before the chamber started showing wear. After that, accuracy drifted but the gun still functioned. For a backup weapon, that's acceptable. For anything else, you'd want to plan on machining a replaceable barrel insert or a hardened chamber liner. There's also the question of what happens when this thing gets dirty. Expedient builds don't have the sealed receiver environments that manufactured firearms do. Dust and debris get in through the ejection port and the magazine well. I carried a small brush and a rag in the same pouch as spare springs and pins. During testing, I fired about sixty rounds without cleaning and only experienced two feed failures, both caused by carbon buildup on the feed ramp. A quick brush pass cleared them. That's not great reliability but it's functional. If you're building this for a scenario where maintenance is impossible, you should expect to clean it after every magazine.

The sound of firing this thing is loud — around fourteen dB higher than a typical 9mm pistol because of the longer barrel and the open receiver. Ear protection is mandatory, not optional. I developed a headache from the concussion alone during my first range session. Second session went fine with plugs and muffs in place. For the firing pin, a simple solid piece of hardened steel about three inches long, pointed at one end, pressed into the rear cap and held by a retaining pin is sufficient. The mass of the pin matters more than its hardness for reliable ignition. A heavier pin transfers more kinetic energy to the primer. I used a pin weighing about forty grams and got consistent strikes across all my test ammunition. Going lighter saved material but increased the misfire rate noticeably — maybe five percent with the lighter pin versus near zero with the heavier one. The tradeoff is that a heavier pin requires a stronger spring to retract it, which brings us back to the spring question. Music wire from the hardware store handles it if you heat-treat it properly. Quench in motor oil, not water, and you won't crack the spring. Water quenching made two of my three test springs brittle and they failed after about fifteen rounds. Overall, building a 9mm submachine gun from expedient materials is very much a doable project if you have access to basic machining tools and some patience. It's not something you finish in a weekend unless you already have parts laid out. The first build took me about two weeks working evenings and weekends. By the third build, I had the process down to about four days. The reliability approaches something workable — maybe eighty-five to ninety percent depending on ammunition quality and maintenance. Not factory-grade, but functional for its intended purpose.