Convert an Air Rifle to a Single-Shot 22LR
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You might have seen in news photographs of African riots--often you can see someone brandishing what appears to be an air rifle. You eyes do not deceive you, it may very well be an air rifle--but it has very likely also been converted to allow firing single-shot 22LR shells, and is not loaded with a pellet!
There exists a method used in poorer African and Asian countries for many years, of converting a cheap 22-caliber airgun to function as a single-shot 22LR rifle. This document does not attempt to give exact instructions because the concept is pretty simple to understand once you have a suitable air rifle in-hand, but rather is just a general run-down of the method.
You need a specific type of airgun for this--just any type will not work. I saw this years ago (sometime in the mid-1990's) in a print magazine about African poachers using converted airguns that were made in India--but at that time, I didn't think much of it because the only similar types of airguns I could easily obtain in the US were expensive European made types, too expensive to be practical to attempt to convert. As of this writing (2004) there is now a huge supply of Chinese-made airguns available (in the US), and some of these are basically identical to the types suitable for conversion this way.
The type you need is a side-cocking spring-piston rifle, with a fixed barrel and a retracting chamber--but the chamber has to be concentric with the bore. As an "example" of what to look for, search for photos of an RWS/Diana Model 52 airgun--this is an expensive European-made air rifle, but it is the general type you need to find to convert. There is a model of Chinese-made airgun that is basically identical in function, if not quite in looks, to the RWS Model 52. The Chinese guns made this way are inexpensive yet constructed very solid--steel action parts, usually in a wooden stock. Totally unlike the inexpensive US-made airguns sold in department stores.
The way you convert the airgun is you ream out a chamber in the breech of the barrel to allow inserting a 22LR shell in it, then you drill a small hole slightly off-center in the piston face, and tap a piece of a nail into the piston and file the nail down so that when the piston reaches the end of the cylinder, the nail sticks out through the air-transfer port in the cylinder and strikes the rim of the 22LR shell and ignites it. That's all there is to it--you don't need to modify anything else on the airgun. The reason other types of spring piston airguns aren't suitable is that the pistons often rotate during use, and the air-transfer ports are drilled off-center, and so there's no way to make certain that the firing pin will always line up and stick out through the air transfer port. Most all "barrel-cocking" air rifle for instance cannot be used because of this reason--the barrel and air-transfer port are not concentric with the piston cylinder.
There are a few technical problems you may have to find ways to deal with, that I will explain here:
First is that in the few examples of these airguns I have seen, the barrel was press-fit into the forend of the receiver and couldn't easily be removed--so you may have to find a way to grind out the chamber while the barrel is still attached to the rest of the gun. One method that has been used successfully is to insert a piece of metal rod a couple inches longer than the barrel and nearly as big as the bore, glue a piece of sandpaper around the breech-end if it with instant-drying cyanoacrylate glue, and then attach a drill to the muzzle end of the rod (left sticking out) and pull on the drill, pulling the rod a short way into the barrel while running the drill. You also need to cut a small groove for the rim of the shell, but that can be done easiest with an electric engraver by hand, from the breech end. The rim groove doesn't need to be perfect; the gun will probably not seal perfectly anyway so you will see some gas escape from the breech--but the breech is set far forward of your face, so the danger is minimized.
The other problem you might have is that you may have to disassemble the gun to drill a hole in the piston. You can try drilling a hole off-center through the air-transfer port without disassembling the gun, but you will need a right-angle attachment for the drill that is small enough, and a drill bit that is short enough, to be used through the gun's loading port, and the piston may catch on the drill bit and only spin inside the cylinder. This is because many spring-piston airguns use pistons that are "floating", in that they are made to allow rotation inside the cylinder. There is a technical reason for this, but it's not important here.
The "sub-problem" with disassembling the airgun is that the wire-coil spring inside is already compressed quite a lot just to get the airgun to fit together. The spring may be pressed into a space only 8-10 inches long, yet may be 24 or more inches long when it is removed from the gun.
You can probably get the airgun apart without any special tools but you will probably not be able to get it back together without them. And even taking it apart without using a spring-compressor is risky, because when you take out the last bolt, that spring will FLY out at high-velocity, throwing various parts with it. To put airgun springs back in (as well as take airguns apart safely) airgun enthusiasts make tools generally called "spring compressors". There are plans online for various types of these that you can make pretty easily, so I won't go into them here except to say that all they are is some type of clamping/guide system to allow you to cram a strong two-foot spring down into a one-foot space, using only your own two hands.
One concern you might have about drilling out the chamber using a steel rod passed "through the barrel" is that you will ruin the barrel. This isn't really a concern, because you won't rub off all the rifling from inside the barrel, and a 22-caliber airgun bore is generally a bit smaller than a 22LR firearm bore anyway. Reaming the entire airgun bore out a bit would actually help lower pressures and improve bullet velocity.
What about other calibers?
The only calibers that are really worth considering are those that might be fired from a airgun-sized bore. Airguns typically come in .177, .20, .22 and .25 calibers, but I have only seen the inexpensive Chinese airguns suitable in .177 and .22 calibers. 22LR is the closest caliber that will require the least amount of grinding to create a chamber.
22WMR uses a case diameter that is slightly larger than 22LR, so that "22-mag" cases can't be accidentally loaded partly into a 22LR chamber at all. I suspect the airgun barrel would hold up to firing 22-mags, but you would have to do quite a bit more reaming to get the cases to chamber and along the entire length of the barrel. And such a gun wouldn't work well with 22LR bullets anymore, due to gas blow-by in the larger-bore barrel.
There also exist now two newer .17 rimfire calibers, and you might be wondering if these could possibly be used with a .177-caliber airgun. The answer is yes and no--yes because you COULD rig up a (solidly-built) 177-caliber airgun to fire such shells and the bullets would certainly come out, but no because it wouldn't work well. First of all, it would be a lot of grinding to cut the chamber, because (unlike the 22-caliber rimfires) the chamber size is a lot larger than the bore size. The second reason it wouldn't work well is because the 17-caliber bullets are quite a bit undersized--the bullets are .17 caliber, and the airgun bores are .177 caliber. You can take a .17-caliber loose bullet and stick it in a .177 airgun barrel and tilt the barrel down, and the bullet will slide right on through and fall out. If you shake the airgun with the .17 bullet loaded, you can hear the bullet rattling inside the barrel.
Is all this safe? Yes -- the airguns used are built very generously of solid steel. The barrel is often as thick as a regular firearm barrel would be, and then the breech end of the barrel is set into a metal block, reinforcing the breech end even further. And finally, the shell is set way ahead of your face--unlike with a conventional bolt-action firearm. If you inspect some cheap zinc pocket pistols, you will see that they are SOLD as firearms, yet are built using a weak zinc frame surrounding a much-thinner steel barrel tube.
After the airgun is converted, it is still fully-capable of firing regular airgun pellets the usual way. And surprisingly enough, this whole concept is not new--back roughly 100+ years ago, combination guns/airguns were commonly sold that were based on this very same concept: spring-piston airguns using a firing pin on the piston face to allow firing rimfire ammo. Many were even constructed small in "children's" sizes, and intended to fire 22-short ammo for outdoor use as well as pellets for indoor use.