by Tom Gaylord
Writing as B.B. Pelletier

Hammerli trainer
The trainer is a complete spring-piston air rifle that slips inside the K31 action and barrel. It interfaces with the K31’s trigger, but uses its own self-contained cocking system.

A history of airguns

Part 1

This report covers:

  • Description
  • Spring-piston
  • Installing the trainer
  • Maintenance before operation
  • Barrel bushings
  • It doesn’t work — yet
  • Construction

The Hammerli trainer arrived this week and I want to tell you about it. This will be the first detailed description I have ever seen of this rare and interesting military trainer. For me this is the airgun equivalent of seeing the dark side of the moon for the first time!

I was surprised how small the box was. And it is very good condition. There are no markings on it beyond what you saw in Part 1, but on the inside there is a cardboard insert to hold the trainer securely. The man I bought it from packed everything very well, and nothing was damaged in shipment. When something is as rare as this, having the box adds a significant amount of value, so I was thankful that the seller went to as much trouble as he did.


The trainer is shorter than I imagined. I knew the entire mechanism had to fit where the K31 bolt was, and the barrel had to fit inside the 7.5mm (roughly .30 caliber) bore of the K31, so that didn’t come as a surprise, but I expected some things that didn’t pan out.

For starters, I had envisioned that the trainer barrel would go all the way to the end of the rifle barrel, and would be held in place by a compression nut at the muzzle. I thought that had to be done to stabilize the barrel for accuracy. But it turns out that the brass trainer barrel measures just 14-1/2-inches from the end of the blued steel receiver. There is a little more of it that’s inside the receiver and I will try to get that measurement for you, but for now this is as far as I can go. So it ends deep inside the 25-inch rifle barrel.

Overall length from the back of the device to the end of the barrel is just over 22-1/2-inches. A lot of it sticks out the back of the rifle when it’s installed, but the operating parts are entirely contained within the unit. Nothing protrudes when it is cocked.

The device weighs 1 lb. 10-1/8 oz. The bolt and magazine of the K31 rifle that had to be removed for installation weigh 1 lb. 4-1/8 oz., so the trainer adds exactly 6 oz. to the weight of the rifle.

The smoothbore barrel is brass, and its breech extends back into the receiver of the device to interface with the spring-piston mechanism. Not only is the brass part at the back the breech of the trainer, it is also part of the loading mechanism, for according to W.H.B. Smith, this is a 6-shot repeater. Until I remove the barrel we won’t know what the breech looks like or how much longer it is.

The rest of the parts are made of steel that has been polished and blued. Some of them, like the retaining clip that holds the device in the rifle, are matte. The cocking handle has two mottled red and brown plastic knobs for grasping to cock the action. Remember — the K31 is a straight-pull bolt action rifle and the trainer also operates that way.

cocking handle
The trainer’s bolt has two plastic knobs like the firearm bolt.


This trainer is actually a spring-piston BB gun. I don’t know if it has the typical Daisy Red Ryder type of BB-gun plunger that’s both a catapult and a spring piston powerplant or if it’s a straight spring piston, but I might find out.

See the mainspring? And also see what appears to be the rear of the piston? Hopefully we will find out what’s inside this trainer someday.

Installing the trainer

I asked my buddy, Otho, if I could borrow his K31 for this test. When he brought it over, the device was there and I showed it to him. He was excited to see how it fit into the rifle, so we decided to try to install it. I removed the K31 bolt and slid the device into the rifle, but it stopped before it was all the way in. Otho suggested the rifle’s magazine might also have to be removed, so I dropped it out and then the trainer slid all the way into the rifle.

K31 bolt
This is the K31 bolt and magazine that have to be removed for the trainer to be installed.

The top latch on the trainer reaches over the K31 receiver and grabs the rear edge of the receiver as it snaps over-center to rest tightly against the top of the device. The installation took about 1 minute the first time because I went slow to avoid damaging anything. After that it took less than 10 seconds to install. And it fit like it was made to. Which it was, of course. This trainer was made by Hammerli (note to European readers — I understand there is an umlaut over the letter A in Hammerli’s name, but if I spell it Haemmerli I will confuse many of my American readers), a Swiss firm that has long been known for maintaining the absolute highest standards in firearm manufacture. Their machining is so precise that “made like a Swiss watch” is a common phrase we use when describing them.

This trainer is also made like a Swiss watch! The parts are so exactly made that they still operate and interface perfectly — even in a rifle selected at random and used for installation 60 years after the device was built! Here is how I would describe what I’m seeing. Harley Davidson motorcycles are/were built to last and are made of strong, rugged materials. An M1 Garand is built to last and to operate under extreme conditions. A BMW motorcycle is built to last and is precisely constructed. And they all could learn lessons from a Hammeri firearm — and from this trainer. It is small, strong, and still functions perfectly all these decades after it left the factory.

Maintenance before operation

Once installed I began to think about shooting it, but I knew it had not been maintained for a long time — probably many years. Based on the date of manufacture and the low velocity it was intended to produce, I felt there was a strong possibility that the piston seal is leather. Leather seals will last a long time if they are oiled, so I put 10 drops of Crosman Pellgunoil into the breech where the BBs are loaded. Then I stood the rifle on its butt overnight. I know the oil has to move down into the place where the piston seal is, and the dry leather will soak it up like a sponge.

If the seal is synthetic, there is a good chance it will not be harmed by the oil, either, so this was a conservative move. It was better than firing the gun with a bone-dry leather seal that might grab and tear.

Barrel bushings

The barrel has two black bushings around its outside. I would guess their purpose is to protect the rifling of the rifle’s bore. I didn’t know what they are made of, but examination under a jeweler’s loupe reveals they are a tiny cord wound around the brass barrel in two shallow grooves that have been machined into the outside of the barrel.

barrel bushings
The barrel is wrapped with some kind of thread to protect the rifling, I’m guessing.

It doesn’t work — yet

The next day I loaded a steel BB and tried to shoot it. Everything worked as it should, but the BB did not come out of the gun. So I removed the trainer from the rifle and inserted a long cleaning rod into its barrel at the muzzle. About halfway down I felt an obstruction that loosened enough for me to push it back to the breech. When it was as far as I could push it, I noted it was still several inches from the breech, so there is no doubt there are a column of things inside that need to come out. The barrel will have to be removed and the objects pressed out with a cleaning rod.


This trainer is 1950s technology and also made by Hammerli. So it’s put together the right way and should be straightforward to disassemble and fix. We shall see, because that’s the next thing I will do.