by Tom Gaylord
Writing as B.B. Pelletier

The B3 underlever from China.

Part 1
Part 2
Part 3
Part 4

A history of airguns

This report covers:

  • Remove the stock
  • Anti-beartrap
  • The piston and sliding compression chamber
  • Removing the mainspring
  • Removing the anti-beartrap device
  • Compression chamber and piston out
  • Cleaning
  • Assembly
  • The trigger
  • Lubrication
  • Does it work?
  • Next
  • Summary

Today I take the Chinese B3 underlever rifle apart and we see inside. This will be a good one.

Remove the stock

The first step is to remove the stock. That’s two forearm screws and one in the triggerguard. Once out, I could see this rifle is very rusty. It’s so rusty that it will take many hours to clean. I don’t have that kind of time so I’m going to clean only what needs to be cleaned.

B3 action
With the action out of the stock you can see rust everywhere! Notice the flat bar in front of the trigger housing. That’s the anti-beartrap.


The rifle has an anti-beartrap that’s designed to prevent the gun from firing unless the underlever is fully forward. This one is very simple, but it does have to be removed before the gun can be fully disassembled.

The parts are held to the gun by a plate that contains two small coiled springs. They make the mechanism work when the underlever is returned to the stowed position. One screw holds the plate to the spring tube.

B3 beartrap
The anti-beartrap mechanism is held to the spring tube by a plate with one screw.

B3 beartrap1
Here is a different view. Sorry for the blur.

The piston and sliding compression chamber

Both the piston and sliding compression chamber are held in the spring tube by a single cross pin. The design of this rifle was starting to look familiar at this point.

B3 cross pin
That cross pin passes through both sides of the spring tube and also the end of the spring guide, which is the silver inner circle. This is what holds the action together.

Removing the mainspring

The spring guide is under a lot of tension from the mainspring, so the action has to be installed in a mainspring compressor for this disassembly. Since the B3 I’m working on has no dovetails for a scope, I can only use a Sun Optics mainspring compressor for this job. Mine is the much older B-Square design that Sun Optics upgraded.

B3 in compressor
The B3 barreled action is in the mainspring compressor.

There has to be a way to bear on the end of the spring guide without bearing on the spring tube. I found a 13/16ths socket that did the job well.

B3 socket
This detail view shows the socket that’s pushing on the end of the spring guide. The object is to relieve tension on the guide so the cross pin (arrow) can be removed. The compressor’s cup that holds the socket turns freely on a bearing, so alignment is not important.

Once there was pressure on the spring guide I tapped the cross pin and knocked it out of the spring tube instantly. All I had to do then was back off on the mainspring compressor and the mainspring relaxed.

B3 mainspring relaxed
The mainspring is completely relaxed. This B3 spring has almost 2 inches of preload!

Let’s look at the parts that have come out of the gun so far. There is the cross pin that held the spring guide in the spring tube, the spring guide itself and the mainspring.

The spring guide is two parts, with a sliding guide that fits on the base of the spring. It does not go inside the spring, but is rounded on the end to center the spring.

B3 mainspring and guide
The mainspring and spring guide are out of the gun. The spring guide is comprised of two parts — a rod to guide the spring and a sliding bushing that centers it on the rod.

The mainspring is relatively straight and seems to be in fine condition. It doesn’t need replacement.

Removing the anti-beartrap device

Next I want to remove the piston and sliding compression chamber, but before I get to them the anti-beartrap device has to be removed. Unscrew the single screw and lift the plate and its springs off the spring tube.

B3 anti-beartrap plate
Before the piston and compression chamber come out the anti-beartrap device has to be removed.

B3 anti-beartrap plate removed
The anti-beartrap plate and springs are removed. This is what pushes the blocking bar into the trigger assembly until the underlever is returned to the stowed position.

Now the underlever can be disconnected from the action. The cocking link is attached to it, and, once the lever is free of the gun, the cocking link can be disengaged from the sliding compression chamber.

B3 remove underlever
That one screw with the sling swivel attached is what holds the underlever to the rifle. Remove it and the underlever comes free of the rifle — allowing you to disengage the cocking link from the sliding compression chamber.

Compression chamber and piston out

Once the cocking link is disengaged from the sliding compression chamber, push out both the compression chamber and piston. In this rifle that was hard. The rust made everything tight and sticky. Lubrication will make a big difference here!

Once the two parts were out of the rifle it was possible to examine both closely. The piston is made of sheet steel, rolled and welded into a cylinder. It was covered with rust. The compression chamber seems to be made of stainless steel, or at least a steel that has a high chromium content. It appears to be formed or machined because there are no welds visible. There was a small amount of surface rust on the patch that’s exposed when the chamber is forward, but the rest of the chamber was rust free inside and out.

B3 chamber and piston
The sliding compression chamber is relatively rust free. The piston, on the other hand, is rusted all over.


I said earlier that there is no way I have the time to clean all the parts as well as I should. So I concentrated on those that mattered. The piston was covered with rust, so I sanded it all over with 240-grit sandpaper and then coated it with Crosman Pellgunoil oil to stop it from rusting further.

The piston seal seems to be made from leather and is in pretty good shape. That surprised me because this rifle looks like it never got a bit of attention. I coated it liberally with Crosman Pellgunoil.

I sanded the little rust from the outside of the compression chamber. The inside was clean and, believe it or not, very smooth. After cleaning off the rust I oiled the outside of the chamber.


Now it was time to assemble the B3. The piston slid inside the compression chamber with the cocking slots in both aligned, then the compression chamber was put back inside the spring tube. Alignment is achieved when the compression chamber fits over the breech.

The trigger

I haven’t mentioned the trigger up to this point. Many articles recommend disassembling it to get the chamber out of the gun and again to install it. I found that wasn’t necessary, but I did have to remove the rear pin so I could pull the sear out of the chamber’s way both for disassembly and again for assembly. And that was when I figured out that the B3 is copied from the BSA Super Meteor! The welded sheet metal piston and the trigger gave it away. You can see what I am referring to in my 9-part report on the Super Meteor. The trigger is shown in Part 2.

The cocking link was then connected to the compression chamber, remembering to slide the anti-beartrap bar over the link. Then the underlever was secured to the spring tube and the anti-beartrap plate and springs was reattached to the spring tube.


I greased the mainspring with Tune in a Tube grease, then slid it back inside the piston. Remembering from past jobs that it doesn’t take much Tune in a Tube, I used less than I would have. Each coil was greased inside and out with very little extra remaining on the spring.

Both parts of the spring guide were greased, then the guide was positioned inside the mainspring. Now the gun was installed in the mainspring compressor and the socket was positioned to push on the base of the spring guide. It took some care, but the guide when in the first time and the cross pin was put in its place to lock the action.

Put the plastic end cap back on the spring tube and then drop the barreled action into the stock and tighten the screws. The B3 is back together. The whole job as described here took 90 minutes, including pictures, cleaning and lubrication.

Does it work?

I cocked and fired the gun several times after assembly — to ensure everything was working as it should. This minor “tune” has transformed the behavior of this B3. It went from a noisy rough-feeling rifle to a quite one that shoots without vibration. There is still a jolt when the gun fires, but it’s reduced from before.

The trigger was already feeling good before I did anything. I did not really lubricate the trigger parts, but my hands were greasy so they picked up something from just that.


I need to retest the velocity, for a before/after comparison. After that I want to shoot it for accuracy again.


This is a job anyone can do, but it does take the right tools. You’ll need some kind of mainspring compressor, a screwdriver set, a pin punch set and a plastic hammer. I think a small tactical flashlight is handy for inspection purposes.

Is it “worth it?” Yes, if your goal is to see the inside of a B3, it is worth it. Also if your B3 is powerful and accurate like this one, this work will make it so much better. Remember, I only cleaned the bare essentials in what I did. 95 percent of the dirt and rust still remain. However, removing them will probably not give you any more power. Peace of mind, perhaps, but not f.p.s.