Adventures in shimming

7% gain in speed and a 14% increase in energy for under $2.

In my last post, I gained 25 fps with 7.9 grain pellets in my Umarex Fuel by simply replacing the breech seal. I left off with the idea that a shim under the seal might boost my speed a bit more. Today, I’ll share my shimming results.

I started by measuring the original shim:

Thickness of factory shim
Thickness of factory shim

To make my own shim, I started with a 9.5 mm ID washer. I centered it on a bolt and held it in place with a nut. The bolt was then chucked into a drill, and I held the washer up to a bench grinder to wear away the edge down to 14 mm.

From a washer to a shim
From a washer to a shim

The shim was then removed and the burr cleaned up with the grinder. The next step was to carefully reduce the thickness of the washer from 55 thousandths down to 20 thousandths. I used 220 grit sandpaper for this. I simply rubbed the shim in a circular fashion against the paper, occasionally rotating to keep the removal uniform. I checked often for thickness, until I reached my goal:

Final thickness of homemade shim
Final thickness of homemade shim

Now for the final result. With 7.9 grain pellets, I now get a maximum of 960 fps. The 10.5 grain pellets topped out at 805 fps. This means I had an overall increase of 60 fps with 7.9 grain pellets and 55 fps with 10.5 grains. Roughly a 7% gain in speed and a 14% increase in energy for under $2. Based on my experience, I would highly recommend checking the seal protrusion on your spring piston airguns.

Shimming can lead to an additional problem – more pronounced barrel droop. Barrel droop occurs when your barrel points at a slight downward angle from the action. If you are using iron sights attached to the barrel, there will be no problems. If you are using a scope mounted to the action, you will probably be shooting a bit lower, and on some airguns, can even run out of adjustment on your scope. In a future post, I’ll share how I work around this problem.

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Author: admin

I've enjoyed shooting and hunting with airguns since my early teen years. For over ten years, I have shared my passion for airguns on this website.

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