Sharp GR-75 repeating CO2 rifle (.20 cal)

Sharp GR-75 reseal 

With thanks to Louis.

Louis says:

I recently went to England for holidays, and picked up a few airguns that I had bought there over the last year.
Including a Sharp GR-75, which is a .20" caliber 5 shot repeater, working on 2 CO2 powerlets.

The magazine of this example was missing, but luckily my friend Shimizu San found a few in Japan.
I think this gun was never disassembled. An estimated 15 of these were sold in the UK in the eighties (by Manchester Airguns).
They weren't popular because the magazine had a pellet fussy reputation.

It took some courage to get my GR-75 apart, but yesterday the conditions were ideal: my ladyfriend was out all day, so I could focus from morning until evening on the project.
The gun works now, on several types of ammo, including JSB.

Iirc, @oldcoot kindly sent me scans of original Sharp drawings that were sent by Kensuke Chiba to Myron Kasok.
One of these documents describes how the magazine pellet holder can be bent to the correct position, using heat from a flame.

It was a matter of getting the right setting for the JSB pellet, which has the right length for the brass pellet arm (6 mm).
Plus cycling must be done with the gun in horizontal position.

I haven't done a shot string yet, but first results are 548 ft/s with 13.73 gr JSB. That's 9.2 ft/lbs, which sounds fine.

One thing about this gun is a mistery to me.
The valve body contains two valves. The exhaust valve, and a small extra valve.
See photos and drawing.

The "tunnel" for this small valve becomes wider towards the brass retaining nut. Perhaps this is meant to release some pressure at the start, to get a flatter power curve?
There was a weak gas leak from that area in my gun just after filling, which stopped after 2 minutes or so.
But then I thought that CO2 pressure is constant when liquid CO2 is present. Hmmm....

The green part is the brass nut. Just after the brass nut, the "tunnel" gradually narrows down to the diameter of the o-ring on the piston.
I don't know what part 101 is. There isn't an l-shaped part in my gun at that location, unless it is technical drawing language for a screw.
There is a screw somewhere in that location, but the piston can move freely out of the valve housing (it isn't blocked by the screw). If this all makes a bit of sense lol.

Pellet cycling

And a photo of the gun just after I picked it up in Lincolnshire.

the extra valve appears to be part of the magazine system.
When the gun is under pressure, the small valve stem pushes against a steel arm that consequently opens up the magazine, so that pellets can be cycled. When there is no CO2 in the gun, the magazine is closed, and pellets cannot be loaded.

I didn't have the correct size of o-ring for this extra valve (about 4 mm OD and 2 mm ID), so I decided to reduce the size of the smallest NBR70 o-ring I had, using my Dremel and 2000 grit sanding paper. It seemed to work first, but with the third charge, the gun developed a leak from this o-ring.

I decided to remove the extra valve, and Loctite a grub screw (5 mm) to close this hole.
Instead of ordering the correct size of o-ring.
This system is, in my humble opinion, a nice example of over-engineering.
The gun is now CO2 tight and the magazine feeds well; I have adjusted the plastic lip, so that pellets can exit the magazine anyway.

JSB 13.73 gr works really well. No jams in the last 50 shots.
Now I am experimenting with hammer springs. The original one is much to strong, giving about 20 consistent shots with the 2 CO2 powerlets.
I tried a weaker hammer spring, which increased the shot count to 30 (at 525-560 ft/s), but there is still lots of hammer bounce.

I might shorten the original spring.

 

Magazine system:

Without the magazine: 

When the little valve stem comes out due to CO2 pressure in the valve, it pushes against the vertical steel part, which then moves back and opens up the magazine, so that pellets can be fed into the gun. 

I've removed the small valve, and Loctited a 5 mm hex screw to close the hole: 

I have mimicked the magazine opening system by glueing a small piece of leather on the vertical steel part. 
This has the same effect as the small valve sticking out under CO2 pressure. 
It is now possible to load a pellet in the barrel when there is no CO2 in the gun, but I prefer this to having an extra potential leak area.