by Tom Gaylord
Writing as B.B. Pelletier

A history of airguns

Part 1
Part 2

This report covers:

  • Seemed simple
  • The back door
  • Use penetrating oil
  • Pop the end cap loose
  • Gun comes apart
  • The piston
  • Constriction cleared!
  • What’s next?

Today is the day we tear into the Hammerli trainer for the K31 Swiss rifle. Before I took on this project I studied the trainer carefully for several days. I wanted to know if there were any surprises in store when I tried to disassemble the mechanism. The Swiss have a saying, “Never use one part when five will do.” That’s my way of suggesting that the Swiss engineering can sometimes be overly complex.

I had already determined by running a cleaning rod down the muzzle that there was an obstruction of over an inch in the barrel at the breech. Experience told me it would be several BB jammed together, with perhaps some other things that shouldn’t be there. It had to be pushed out.

Seemed simple

My examination revealed that the design of the trainer powerplant appeared very straightforward and even simple. Let me show you what I saw.

The problem was a constriction inside the barrel. The easiest way to clear it would be to take the barrel off the trainer and push the constriction out. The barrel is held on by two very small slotted screws at the back periphery. The screw heads looked a little messed up when I examined them, so I put on a magnifying hood and looked closer. Lo and behold, both screws are staked! And the stakes are deep. Someone really did not want this barrel to come off this trainer under normal circumstances!

staked screw
That deep stake in the screw head prevents it from turning. Try to loosen it and you will likely bugger the slot.

The back door

The other way to clear the barrel would be to strip the powerplant of all its internal parts and push the BBs out with a rod through the muzzle. With nothing in the way they should come right out. But was the trainer as straightforward as it appeared, or were there surprises inside, just waiting for me?

After examining the action it seemed like the easiest thing to do was to remove the end cap and then the mainspring and piston. If this action really was straightforward, that would leave the spring tube hollow and allow the BBs to be pushed out of the gun.

end cap
The end cap has a narrow knurled band (arrow). It appears that should be used to unscrew the cap and remove the mainspring. But that sear bar  on top (top left in this picture) may be a problem.

sear plug
This view shows the plug (arrow) that holds the sear spring and sear in place. Remove that screw and the plug should come out with its spring.

I removed the screw that holds the sear plug in place and removed the plug and screw. That gave me a better view of the end cap.

sear plug out
With the sear plug and spring out of the gun I could clearly see the end cap threads. The sear bar is still captive.

Once the sear plug and sear spring were out I could see the threads that held the end cap in the action. It appeared that all I had to do was clamp onto that knurled ring of the end cap and unscrew it from the main tube. I tried it first by hand with no success. Then I grabbed the knurled edge with cannon plug pliers (pliers with curved jaws that have nylon pads in them) and tried again. Also unsuccessful. I had to use more force, and this was getting into a dangerous area where I could mar the finish or damage the trainer in some way. So, I pulled out my secret weapon — my buddy Otho.

Use penetrating oil

He told me to put some penetrating oil on the exposed threads of the end cap and to keep on doing it for a day. I used Kroil, a product I have a lot of faith in.

Otho was a turbine engine mechanic for the Army and has forgotten more about fine maintenance and careful work than I will ever know. So I took the trainer to his house and we set about working on it. When I say “we,” I mean I did whatever he told me to.

Pop the end cap loose

First, he clamped the knurled ring of the end cap in his vise that has copper jaws. I grabbed the action and tried to turn it. I got a bump that felt like a release, and when we examined the knurled ring there were no signs of slippage. The end cap had broken free! That Kriol I used had done its job.

Next he grabbed the knurled ring with a pair of cannon plug pliers and tried to turn it. It was stiff, but it did turn. After a few turns it got much easier, which was the Kroil kicking in.

turning end cap
Otho turns the end cap with cannon plug pliers.

Finally the cap got so loose that Otho was about to unscrew it the rest of the way. When it released the mainspring was under some tension and popped out about an inch.

end cap out
The mainspring is under this much tension.

Gun comes apart

Now the mainspring came out with the end cap. The sear bar was also freed and slid out of the action. Now it was time to remove the piston. I stuck a screwdriver blade into the cocking slot of the piston and pushed it to the rear. It was very tight, which in this case is a good sign. After a bit of fiddling it came out and we can now examine it.

The piston

The piston is a similar to the plunger than’s in a common Daisy BB gun. It looks partly like a standard pellet rifle piston but there is one important difference. It has a long air tube in front that pushes the BB out of the breech and up the barrel, accelerating it to around 50-80 f.p.s. When the piston gets to the end of its travel, compressed air is forced through several small holes at the base of the air tube and travels up the tube to get behind the BB. That compressed air is what blasts the BB to its top velocity, which in this case is around 200-250 f.p.s. I’m just guessing at this, because I haven’t tested this one yet, but in Smith’s book he put the velocity at 250 f.p.s.

Here is the piston out of the gun.

The piston seal is leather, like we all thought. It looks very good, but it’s definitely dry. I need to soak it in household oil for a couple days before putting it back in the gun. That gives me time to clean the compression chamber.

We noticed that the air tube was bent. It would not go all the way into the muzzle without a hard push. That means it’s doing the same thing at the breech, were it lives. And that means power will be lost to excessive friction.

air tube bent
The air tube is bent. That’s not good.

Undoubtedly the tube was bent from slamming into the breech constriction repeatedly. It had to be straightened, but because it is hollow we ran the risk of snapping it off and having to start from scratch. There are no replacement parts for these trainers. Otho carefully applied sideways pressure and straightened the tube until it slid into the muzzle easily. Problem solved!

Constriction cleared!

I used a .177 cleaning rod down the muzzle to push the constriction back through the now-empty spring tube. And I found exactly what I expected to find — seven steel BBs and three pieces of lead — one grossly oversized. The steel BBs all have flat spots on them from repeated blows of the air tube, as shot after hopeful shot was attempted over the period of who-knows-how-many years.

These steel BBs and pieces of lead were lined up in the breech of the trainer.

The last picture I’ll show you is of the leather piston seal and the air holes at the base of the air tube. Those familiar with BB-gun construction will see the resemblance right away.

air tube
Here you can see the leather piston seal and the air holes at the base of the air tube.

What’s next?

The next step is to clean everything, soak that piston in oil and then assemble the trainer. After that I fully expect it to function as it is supposed to.