by Tom Gaylord
Writing as B.B. Pelletier

A history of airguns

Part 1
Part 2
Part 3

This report covers:

  • Famous last words
  • A month of frustration
  • Found on the floor!
  • One ball or two?
  • Stacking BBs
  • August to the rescue!
  • Bearing found!
  • Assembly instructions
  • Performance
  • Velocity — Air Venturi Copper-Plated Steel BBs
  • Hornady Black Diamond BBs
  • H&N Smart Shot lead BBs
  • Daisy Avanti Precision Ground Shot
  • Fixed again!
  • Trigger pull

Famous last words

With the help of a link from reader August, I finally figured how to correctly assemble the Hammerli trainer for the Swiss K31 Schmidt Rubin straight-pull rifle. Taking it apart was no problem whatsoever, and you read about that in Part 3. Once the powerplant was apart I was able to push the jammed BBs and lead particles out of the bore. Then I cleaned the powerplant, straightened the air tube and soaked the piston seal in oil for a couple days. Then I thought I would assemble it and continue the test. In fact my last statement in part 3 was, “After that (assembly) I fully expect it to function as it is supposed to.”

A month of frustration

Yeah — right! It took nearly a month of fiddling with the parts before I learned their secrets and was able to get the trainer back to operation. My buddy, Otho, and I spent about 2 hours one day just assembling and disassembling the thing; wondering why it didn’t work. One thing became obvious to both of us. Something was missing! In all likelihood it was a small ball bearing.

Found on the floor!

Otho swept the floor of his gun room with a powerful magnet and collected a tiny ball bearing that measured 0.156-inches in diameter. This is where knowing something about airgun ammunition comes in handy, because no common airgun ammo is that size. A steel BB measures 0.171-0.173-inches in diameter — enough larger that you can see it with the naked eye when holding both the bearing and BB in your hand.

And this tiny ball bearing happened to fit in every machined channel and hole in both the sliding sear bar and also in a slot cut directly into the gun’s piston. A steel BB was too large for these slots and holes. That told me that the ball bearing Otho found was more than likely a part of the powerplant that had fallen out when I disassembled it.

Hammerli trainer sliding sear bar
The sliding sear bar is machined to fit a small ball bearing exactly!

Hammerli trainer ball piston
The piston is also machined to fit the small ball bearing! When the powerplant is assembled, this slot is directly against the machined slot and groove in the sliding sear bar shown above. Not by coincidence!

One ball or two?

Try as we might, though, the ball bearing did not fix the problem. The trainer still didn’t work. But something special did happen! At the end of our 2-hour skull session, Otho’s eyes narrowed to slits and he mused, “I wonder if there is a second ball bearing we haven’t found?”

At first I thought he was chasing a rainbow. Nobody in their right mind would use two ball bearings in a mechanism like this, because the only way that could work is if the two bearings pressed against each other when the gun was cocked! They would have to be the actual sear of the trainer. Nobody would be crazy enough to do that! That would be like stacking BBs!

Stacking BBs

In the Army we had a number of sayings that we used to convey special meanings. “Two up and one back” refers to a foot patrol where you have two soldiers ahead of the main column and one trailing behind for security. “Hot socks and a dry meal” is a humorous transposition of dry socks and a hot meal — a foot soldier’s two most coveted things when he’s in the field. “Always put an officer in change” means to have someone responsible for every operation, so you can court-martial them if things go wrong.

And then there is the phrase “stacking BBs” as in, “The major’s got me stacking BBs.” Translation: “The major has me doing an impossible task with a futile outcome. — He’s wasting my time.” When you think about it, stacking BBs is one of the most impossible tasks there can be. And yet, Otho thought this trainer might actually work that way.

I continued to ponder the problem and Otho swept the floor of his gun room with a magnet once more. I thought I might have just ruined a very expensive and valuable airgun by my careless disassembly technique. Whenever you have a mechanism you don’t know, try to disassemble it the first time while it’s inside a plastic bag. That works great if you remember to do it. Not so good if you only remember it afterward and write it in a blog.

August to the rescue!

I told you guys on the blog I was having problems with this assembly and, as I mentioned in the beginning, August came to the rescue by sending me to a link where somebody described how to assemble the trainer. Key among his instructions were the words, “…your Trainer is (possibly) assembled without the two steel balls…” So — there were two balls! I called Otho and we both rejoiced!

The link August sent also had a Hammerli assembly drawing that showed the trainer uncocked and cocked. In the cocked drawing you can clearly see the two balls on top of each other and bearing against each other — held in by their respective parts. The sliding sear bar kicks the supporting ball out from under the ball in the piston, releasing the piston to move forward under the power of the mainspring.

Hammerli trainer drawing
The drawing shows the two ball bearings in the uncocked position (below) and cocked (above). When cocked they are literally stacked BBs!

I measured the ball he had found on the floor — the one that fit in all the machined recesses and holes in the sliding sear bar, the piston and even the hole inside the spring tube. It measured 0.156-inch in diameter. The conversion app on my computer says that is 3.9624 millimeters, so I set about looking for a ball bearing that size.

Hammerli trainer bearing
The bearing Otho found on the floor measures 0.156-inches.

Bearing found!

Most bearing companies want to sell you ball bearings in races (captive cages). I wanted just the balls, themselves. Fortunately BC Precision in Chattanooga, Tennessee, sells them that way. They come in packs of 25, but the cost is just $4.95 per pack and the shipping is free. I ordered both a 4mm ball and a 5/32-inch ball. I measured them when I received them and the 4mm ball measures 0.1565-inches. while the 5/32-inch ball measures 0.155-inches in diameter on my dial caliper.

Hammerli trainer bearing replacement
The 4mm replacement bearing measures 0.1565-inches.

Assembly instructions

I followed the online instructions for installing the balls, which I will now give here for posterity.

First, install the piston in the spring tube and slide it all the way forward, so the air tube at its end fits inside the breech of the barrel.

Both balls will be inserted into the sliding sear bar, one at a time. Slide the bar inside its container in the spring tube housing until the hole in the bar aligns with the widened hole in the container slot. Drop the ball into the widened hole, but have your finger under the sliding sear bar to keep the ball bearing from falling through. Now, push the sliding sear bar forward as far as it will go. It may help to invert and even shake the mechanism as you do this.

Hammerli trainer sear bar ball one
Here I’m about to drop the first ball bearing into the sliding sear hole. What you don’t see is my finger that should be inside the spring tube to prevent the bearing from falling completely through all holes. That finger was on the shutter button of the camera.

Hammerli trainer sear bar forward
After inserting the first ball, I slid the sliding sear bar as far forward as it would go — keeping my finger inside the tube to prevent the ball from falling through. You may have to jiggle the gun and fuss as you do this, because the ball has to drop into another hole inside the spring tube. When it does, it is captured and you can slide the sear bar back out for ball two.

Return the sliding sear bar to align its hole with the widened hole in the slot again. Put your finger under the sliding sear bar again to prevent the second ball from falling through. Now push the sliding sear bar with the second ball far forward as it will go — inverting the entire mechanism if need be to accomplish this.

Hammerli trainer sear bar ball two
Now drop ball two into the same hole, with your finger in the tube to keep it fropm falling through. Then slide the sear bar all the way forward again. This time may be more difficult, because this ball has to climb up a sharp shoulder. Just keep fiddling with it, because the sear bar will go forward all the way.

Once the sliding sear bar is as far forward as it will go, hold it there and look into the rear of the spring tube to make sure no ball bearing has fallen into the tube. Once you see it is clear, install the sliding sear bar spring in its position and then install the spring guide, mainspring and end cap.

Hammerli trainer assembled
The sliding sear bar is installed and held by its screw and anchor. The mainspring and end cap have also been installed.


The trainer now cocks as it is supposed to. Up to 6 steel BBs are loaded into the gravity-fed magazine and shooting can begun.

Velocity — Air Venturi Copper-Plated Steel BBs

I loaded 5 BBs at a time and started shooting. First came the Air Venturi Copper-Plated Steel BB. Ten of them averaged 283 f.p.s. The spread went from a low of 278 f.p.s. to a high of 286 f.p.s., which is 8 f.p.s. So, the trainer is pretty consistent.

This is a lot faster than I was expecting. I had thought it would top out around 250 f.p.s. and maybe not much more than 200. Apparently soaking the piston seal in oil was a good thing.

Hornady Black Diamond BBs

Hornady Black Diamond BBs came next. Ten of them averaged 284 f.p.s. with a low of 281 f.p.s and a high of 287 f.p.s. That’s a 6 f.p.s. spread! It’s also very close to what the Air Venturi BBs did.

H&N Smart Shot lead BBs

Next came the H&N Smart Shot copper-plated lead BBs. They averaged 238 f.p.s. with a spread from 233 f.p.s. to a high of 244 f.p.s. That’s 11 f.p.s. I plan to try these BBs in the accuracy test that’s still to come.

Daisy Avanti Precision Ground Shot

The last BB I tested was the Daisy Avanti Precision Ground Shot that is produced especially for Daisy’s Avanti 499 Champion BB gun. The first two shots went out at 285 and 288 f.p.s., then the third shot stuck in the bore. That was the end of the test, as the trainer had to be disassembled and cleaned out. Thank goodness I now know how to do that!

I will not try the Precision Ground Shot again in this trainer. The tolerance is just too close. It ought to work well, according to the specs (0.175-inch steel balls were recommended) but it doesn’t in this one. The stuck BB was about one inch from the muzzle of the trainer’s barrel. The bore is shined to a mirror there, so this was a dimension problem. The stuck BB measures 0.1735-inches in diameter.

Fixed again!

I disassembled the trainer, removed the stuck BBs and assembled it again, taking picturs as I went. This time instead of taking almost a month, it took about 15 minutes, start to finish. I inserted the trainer back into the K31 rifle, pulled the rifle’s trigger to set it for the trainer then cocked the trainer and shot it as before.

Trigger pull

A K31 rifle has the finest trigger pull of any military rifle. Our own M16 with a National Match trigger installed feels like a harrow being dragged on an asphalt road in comparison. The sear breaks at 2 lbs. exactly. And now we know what it means when the sear breaks. The BBs are no longer stacked!

We’ll look at accuracy next. I have been shooting this little gem a lot and I know you’re going to want to see what it can do.