by Tom Gaylord
Writing as B.B. Pelletier

A history of airguns

Part 1
Part 2
Part 3
Part 4

This report covers:

  • Thanks to August
  • A BB gun
  • Gravity feed
  • Clean targets
  • Sight-in
  • Air Venturi Steel BBs
  • Hornady Black Diamond BBs
  • H&N Smart Shot lead BBs
  • Avanti Precision Ground Shot
  • Summary

Today we look at the accuracy of the Hammerli trainer that was made in the 1950s for the Swiss K31 Schmidt Rubin military rifle. It has taken some time to arrive at this point, because the trainer wasn’t working when I bought it. I could tell something was jammed in the barrel, and after disassembly it proved to be several steel BBs and some chunks of lead. But during the disassembly process I lost two ball bearings that play a vital role in the trainer’s operation.

Thanks to August

You can read Parts 3 and 4 to learn what happened to me when I disassembled the trainer. It boiled down to loosing those two ball bearings, then finding one of them and wondering whether there was another one I couldn’t find. Reader August saved the day by locating a web page where someone talked about the assembly of the trainer and that was where I confirmed there are 2 bearings, not just 1.

Part 4 of this report describes the assembly procedure, which is something of a Chinese puzzle box. The two bearings actually form the sear of the trainer and have to be assembled correctly to work. It is an ingenious design that adds nothing to the crisp 2-pound trigger pull of the K31.

A BB gun

Since the trainer is essentially a BB gun, I decided to test it like one. That means BB-gun targets at 5 meters. I rested the rifle on the UTG Monopod rest, which you have learned from past reports is about as stable as a bag rest when done right. It’s certainly steady enough for this test. But it gave me one problem I wasn’t anticipating.

Gravity feed

The trainer feeds by gravity. If the muzzle is elevated slightly the BBs roll back and fall into the breech one at a time when the air tube on the piston uncovers the place in the breech into which one BB will fit. I forgot that when I was shooting off the monopod. The trainer cocks so easily that I left the rifle level and on target every time I cocked. Invariably the fifth BB failed to feed. But when I elevated the muzzle a bit it fed perfectly. You need to keep this in mind when using a gravity-feed airgun.

Clean targets

I wanted to show you how I set up the targets on the UTG Pellet/BB Trap, so the BBs will poke reasonably clean holes in the paper. First I tape the targets to a cardboard backer in the front of the trap, then I use wide shipping tape over the bullseye area on the target. [Note — the tape over the target doesn’t work. I explain that at the end of this report.]

cardboard backer
A sheet of fresh cardboard is slid into the front of the UTG BB trap. Targets will be taped tightly to this.

tape over bull
It’s hard to see, but there is a piece of wide tape over the bull on this target. [Note — this doesn’t work.]


Because the trainer comes out of the K31 easily, I believe the point of impact probably changes each time it’s installed. I had it zeroed a week ago but when I shot this time the BBs were hitting high. The good news is you adjust the rifle’s sights exactly as you would for firearm ammunition. The K31 sights adjust for both windage and elevation. Elevation is by raising and lowering the rear sight leaf, and windage is by sliding the front post from one side to the other.

K31 front sight
Looking down on the front sight we see that it goes left and right when slid in its channel, fore and aft. It’s staked to remain tight in the groove.

Air Venturi Steel BBs

First up were Air Venturi Steel BBs. Five of them went into 1.379-inches at 5 meters. That wasn’t what I expected. I was hoping for a group that’s closer to what the Daisy 499 can do.

K31 Air Venturi group
Five Air Venturi BBs went into 1.379-inches at 5 meters — 3 in the bull and 2 to the right. I expected better from this trainer.

Hornady Black Diamond BBs

Next I loaded 5 Hornady Black Diamond BBs into the magazine and gave them a go. Four of them went into 1.174-inches, which isn’t too bad, but the last shot (it was actually the second in the string) opened the group to 3.11-inches. Not very impressive.

K31 Hornady Black Diamond group
Five Hornady Black Diamond BBs made this 3.11-inch group at 5 meters. Four together and one at the bottom right.

Wow! This wasn’t going the way I expected. Maybe there is more to learn about this trainer. Of course it is true that the shot size should be 0.175-inches, according to author W.H.B. Smith. I thought I had disproved that when I stuck an Avanti Precision Ground Shot in the barrel in Part 4, but we will see in a little bit what else that could have been.

H&N Smart Shot lead BBs

I tried H&N Smart Shot lead BBs next. I thought they would be the last BB I tested, but I changed my mind after seeing what they did. Five BBs went into a group measuring 0.979-inches between centers. While this is nowhere near what the Daisy 499 can do, it does show some potential for the trainer. I think I need to do a little more testing.

K31 Smart Shot group
This is more like it. Five Smart Shot lead BBs are in 0.979-inches. Not great, but certainly better.

Avanti Precision Ground Shot

In Part 4 I said I thought the Daisy Avanti Precision Ground Shot was too large for the bore of the trainer and I would not test it farther. Well, in this test I learned that the way I was holding the rifle (level, so gravity could not act on the balls in the magazine) was causing the problem. So I loaded some Avanti Shot and gave it a try.

Five Precision Ground Shot went into 2.03-inches at 5 meters. That’s the second-worst showing of the 4 BBs tested, and looking at the scattered shots I would see no reason to test this ammunition any farther.

K31 Avanti Precision Ground Shot group
Five Avanti precision Ground Shot landed in 2.03-inches at 5 meters. Nothing to see here. The test is over.


First, I have to observe that putting tape over the target face does not improve the BB hole. In fact it makes it smaller. Just tape the target to the cardboard backer tightly and leave the target face alone.

We spent a lot of time with this Hammerli trainer. Much of that was spent fixing it and learning how it works. It was a challenge, but now we know its secrets.

I don’t know if I will return to this airgun in the future, but I can tell you that I do intend keeping it. I just won  a K31 rifle of my own on an auction site to house the trainer, so I can give Otho’s rifle back to him. What a great journey this has been!