GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: morrodds on July 08, 2013, 07:28:58 PM
-
As it was a nice morning Saturday, I set up a 50 yard range to test some more mods to the Girardoni. All shots taken from a sand bag on a folding table.
(http://i267.photobucket.com/albums/ii311/morrodds/302_45x20_zps0bf5727e.jpg)
Here is a 20 shot string. The first shot was 8 inches high as I had just replaced the valve with a new design. I adjusted for this on the remainder of the shots.
(http://i267.photobucket.com/albums/ii311/morrodds/Girstring_zps752cb22d.jpg)
This is the shot string over the chrono. Pressures, 1000psi to 580psi. Compare this to an initial string from January.
(http://i267.photobucket.com/albums/ii311/morrodds/Hankschrono.png)
Quite an improvement, IMHO.
The following changes were made since January:
The valve head was decreased from .500 to .375. The valve face was flattened from a 90 degree to 110 degrees. The valve orifice was retained at .302 and taper of 120 degrees. I think this reduced 'stiction' of the valve and decreased shrouding. It did improve consistency and increased average velocity. Didn't change the maximum power though.
My ball casting technique has improved resulting in more uniform diameters and densities. The balls are tumbled for ~18 hrs with a wax lube.
The breech cavity has been deepened and polished to allow easier loading. I had been getting 2 or 3 balls jamming in the breech per magazine load before.
The striker spoon had distorted and was binding. I cleaned it up and gave it a little more clearance as mentioned in the Girardoni assembly video.
A bronze piece was soldered to the nose of the tumbler to 'soften' the striking and rebound on the mainspring. See the high speed Girardoni lock video.
I made an adapter to increase the trigger pull distance, drop and cast-off of the buttstock. This makes it far easier to attain good sight acquisition. Hank can now shoot his very comfortably.
Next is practice and more practice. Also, I'd like to try one of those peeps that affix to the lens of my glasses. The sights of the Girardoni are small and difficult to see with my aging eyes.
These videos can be seen at http://tinyurl.com/m8dc4rq (http://tinyurl.com/m8dc4rq)
Martin
-
On average, it looks like you are losing about 1% velocity per shot, and about 2% pressure per shot, in pretty much a linear fashion.... For a gun where the valve / hammer strike doesn't self-regulate that's probably about as good as you're going to get.... WELL DONE !!!
Bob