Recessing the Stock Screw on the Sportsman QB77

Shooting the QB77 the other evening and was annoyed by the stock nut on the bottom of the forend. It's exactly where I want to rest the gun on my offhand. This is, what, the third gun that I've altered this way? I blogged a similar fix on a Crosman 180, but the QB77 stock is shaped differently, and besides, I wanted another approach.

Shot the gun until empty of CO2. Removed the trigger shoe, safety lever, stock nut and "locating stud"--to use the old Crosman lingo for the part.

The QB locating stud is double threaded. M6 x 1mm at the valve end, M7 x 1mm at the nut.

The hole through the stock is about 5/16", and since the gun is made in China it's safe bet they used an 8mm bit.

Forstner bits

A 5/8" is close (or maybe even the same) to the diameter of the counterbore for the washer and stock nut.

Clamped the stock to the drill press table and aligned by guesstimation and pure, unadulterated luck.

Deepened the factory counterbore.

Ran the forstner bit at a fairly high speed to help prevent tear outs in the wood. The bit also needs to be nice and sharp for a clean cut.

Probably a good idea to treat the bare wood with something.

Looks like a pretty good match. After it dried, I gave it a good slathering of Danish oil.

Need to make a couple parts--a screw cup to fit into that counterbore as well as a new locating stud. Gonna use a piece of aluminum for the cup. Normally, I'd use steel and blue it, but polished aluminum will work since the stock sports an aluminum Morgan buttpad. Found a scrap of aluminum rod buried on the workbench.

Turned down one end to 0.621". Dang.

I wasn't paying attention and missed my (self-imposed) target diameter by 0.004". Flipped the piece around and turned the other end to 0.625".

Second time was the charm. Spotted for a through hole. Set the piece aside and switched to making the new stud.

I'm going to take advantage of the 8mm through hole in the stock. Since 5/16" is almost exactly 8mm, I cut the head off a long 5/16" bolt and faced the end.

Threaded one end M6 x 1mm to fit into the valve. Original stud is below. Cut the thread to length.

Cut down and faced to OAL. This is the gun stock end of the stud.

Spotted-n-drilled with a #19 bit about 3/4" deep.

Tapped M5 x 0.8mm. Cut a shoulder at the base of the M6 (valve end) thread. The shoulder was cut to 7.09mm diam.

Back to the aluminum screw cup. Through drilled.

Then drilled a countersink for the head of a M5 bolt.

Locating stud.



Installed. That shoulder is a nice tight fit in the hole through the gas tube --the valve won't ever shift backwards under gas pressure.

Another view. You can see the M5 x 0.8mm threads in the stud.

Sits just below the bottom of the counterbore in the stock. The new 5/16" diameter stud is a snug fit in the through hole. Much better fit than the stock stud.

Left the screw cup a few thou long for fitting to the contours of the forend. A bit of dykem layout fluid...

and the cup is pressed home. Used a scribe to trace around the cup where it stands proud.

Should have used the drill press and a tiny sanding drum, but it was 10PM and I didn't feel like going out to the garage. Yep, lazy. Used a cratex wheel in the Dremel. You can see the scribed line in the dykem.

Getting there. Marked the inside of the screw cup. No way are the stock contours symmetric, so the cup is actually directional.

Sanded to just below the line. A test fit showed a high spot here and there. The cup was gradually worked down until I was satisfied, then sanded out and polished.





Don't think the bolt head is finished to the same degree as the cup. I'll polish it and re-blue when/if I refinish the stock.

It blends well and matches the polished aluminum plates on the Morgan perfectly. Most importantly, my offhand is happy again.

Benjamin 3030 Disassembly, Part 4

Just about done.

Stock screw removed.

The bent c-washer captures the screw so you can take down the rifle for transport.

This is a rough approximation of how it loads.

BBs are inserted into the tube.

When the muzzle is raised a BB rolls into the valve assembly.

Without the barrel in place it’s easy to see how the magnet works.

The BB captured by the magnet.

The barrel at the receiver end. 4 ports for gas flow.

Again, a BB dropped into the hole is stopped by the barrel.

When the rifle is cocked by moving the hammer forward the BB drops down. When the rifle is fired the barrel moves back, picks up the BB and knocks open the valve.

As you can see here, the barrel sticking through the valve chamber would have knocked open the valve and gas would flow through the ports and propelled the BB out the barrel.

I think I may just clean the 3030 and see if I can get it to work before thinking of any refinishing. Now where’s my 5 gallon steel pail?

Benjamin 3030 Disassembly, Part 3

On I go…

I took up the force of the hammer spring by holding the safety (smaller of the two screw headed posts) back and unscrewed the cocking handle.

Again I pushed down on the bench which took up the spring force and allowed me to remove the safety post.

Here’s a shot showing how the end of the barrel protrudes just past the housing.

With the safety post removed the barrel comes out.

Followed by the hammer spring.

The hammer is soldered to the barrel. Notice the revolving cutaway ring, that’s for the safety post and allows it to be rotated into the slot in the housing.

A collar is soldered on the barrel as a stop for the hammer spring, which is captured between it and the front (muzzle) of the housing.

Removing the safety ring. I rotated it and it dropped down.

The o-ring in between the brass and the spring needs to come out.

I pushed down on the spring so I could dig the o-ring out. It was rock hard, which is undesirable in an o-ring.

With the o-ring removed the valve spring can come out.

The now disintegrated o-ring and valve spring.

Then the exhaust valve drops out.

That’s a heck of a piercing pin.

The screen removed. I don’t see how it does much to capture debris?

The exhaust valve face. The flats on the larger diameter allow the CO2 to flow around the open valve.

The piercing pin unscrews.

The seal comes out easily. Not sure what it’s made from. I may try and see if it works before making a replacement.

A typical valve seat inside the valve.

More to come…