Gamo Magnum, Part 3

It has been a while since I worked on this air gun. I was disappointed in the results I got with the replacement O-ring seal I made. When I started I had my doubts as to whether or not it would work and if it did work, how long it would last ? I just wasn't getting a warm fuzzy feeling from this fix.
While driving around yesterday my mind was wandering, thinking about my latest air gun, a Fast Deer (Chinese side lever) Thinking about the leather piston seal it has and how dependable and forgiving they are. Then the Gamo Magnum popped into my mind. Why not build a leather seal for it instead of trying to make an O-ring work or trying to find a supplier of 29 mm piston seals.
I have made a few replacement leather seals in the past with good results and I think this will be an easy fix for this air gun.
To get this project going I built a form from aluminum to give my seal the right dimensions. A 1.112"  recess was milled into a piece of round stock. then a .9" plug was turned to form the shape of the inside of the seal.

The easiest source I have found for leather is at the local Farm and Home store, a tape measure holder. 

I cut out a piece big enough for the seal, soaked it in warm water to soften it up. I also worked some Neatsfoot compound into the leather (mink oil works also). 

With the leather nice and pliable I put it between the two halves of the form and use a C-clamp to force the leather into the form.

 One problem I ran into in the past is the seal comes out with a rounded bottom instead of a nice square corner.

 To make the bottom more square it helps to cut off the excess then use a flat tip screwdriver to force the leather down inside the form. It wont be perfectly square when I remove it from the form but I have found that once it is installed in the air gun and fired a few times the rounded corners disappear.

 The last step is to trim the leather flush with the top of the form. I will leave it clamped over night to give the leather time to dry out and take on the shape of the form.

I turned down some more aluminum to hold the seal in place on the piston and fill up the dead space inside the seal.

Everything went together well. I kept the seal short to keep the swept volume close to where it would be with the correct seal. I added more Neatsfoot compound just before putting the piston in the rifle.

 The following groups were shot at 10 yards from a rest using open sights. I used 3 different RWS wad cutters. They all grouped about the same. Velocities still seem low but I have to remember this air gun was built in the late seventies or early eighties. I haven't been able to narrow it down to an exact year. In that era I believe 700 to 800 fps was a high out put. Regardless these are the results I got....

 Miesterkugeln 8.2 grain averaged 641 fps with an extreme spread of 19.5 fps

 Meisterkugeln 7 grain averaged 758 fps with an ES of 11 fps

Super Magnum 9.3 grain averaged 534 fps with an ES of 26.4 fps. I had to move my POA down as the POI was off the paper with the first shot. 

So far I'm happy with the way it is shooting. I'm going to tear it down again and make sure everything is in good shape. This thing has a great trigger and seems to want to shoot well. Once I'm sure everything is good inside I'll mount a scope and see what this thing will really do.

I tried mounting a scope on it and didn't have much success it was hitting to the right of POA with the windage adjustment maxed out to the left. That will be an issue for another day. While I was at it I took the scope off and shot another group with open sights. I was careful with each shot and was rewarded with this group.

Ten shots into one ragged hole, a nice little four leaf clover. 

I may just shoot this with open sights and forget about using a scope.

I thought I'd add a couple pics of the Magnum as it looks now.....

 To finish it off I just need to make a plug to fill this hole.... Maybe some black plastic round stock or just make it out of aluminum, since I have some on hand.