fl(air) gun - big bore air pistol conversion

[jackssmirkingrevenge]

I thought this deserved a separate thread from the earlier development.



Cartridge and barrel with a Polish flare gun from the 1950s based on the Soviet SPSh-44.



Cartridge disassembled, most parts are machined from 12L14 steel apart from the body that is 6061 aluminum



Operating principle roughly to scale.  The effective area of the piston is just 0.157 square centimeters or 0.024 square inches, but at 3000 psi that's a force of 72 lbs on the ball locks, in hindsight that difference in diameter could have been smaller.



Cartridge inserted in the launcher



Video with a more detailed explanation of the mechanism and some action shots



Test slugs made from 12 g CO2 capsules, the top two have small screws inserted in the nose and lead filling on the inside.

From top to bottom here are the weights and measured velocities and muzzle energy:

19.8g - 277 fps - 52 ft lbs

13.0g - 401 fps - 72 ft lbs

7.9g - 486 fps - 64 ft lbs

Normally more projectile weight gives more energy, but a dynamic at play here is that the projectile tail being hollow means there is more "dead" volume in the barrel, giving the air more space to expand and therefore dropping pressure.



Aluminum dart with a steel core, weight is 20.7 grams but I didn't measure the velocity, presumably it's below 300 fps.

[Rob M]

love the slug variety ! 72 fpe is unreal on this compact platform.. a long version will be interesting also.

[Back_Roads]

 This guy is getting what I thought to be some good energy from a plastic gun  

[Mr.P]

Higher power and lower pressure sound like a good combination.

[Rob M]

question , is the exhausted air expelled past the firing valve stem?

[jackssmirkingrevenge]

a long version will be interesting also.

For legal reasons it has to stay below 500 fps so unfortunately I probably won't be pushing it to its limits in terms of barrel length.

some good energy from a plastic gun

I love the SPAS-12 looks, interesting candidate for a bit of an upgrade.  I'm guessing it works like the APS shotguns, a mechanism similar to the old Nelson paintball markers:

question , is the exhausted air expelled past the firing valve stem?

I'm not sure I understand, there is no firing valve per se so it's not an exhaust valve.  The piston is held in place mechanically by the steel balls, and that part of the mechanism is sealed off from the chamber.

[Hoosier Daddy]

Very interesting!
For quite some time I have been contemplating the Umarex T4E HDS Shotgun .68 to play with here on the forum.
It is CO2 powered and there are  projectiles and molds for this that I believe would be allowed here.

[Back_Roads]

Very interesting!
For quite some time I have been contemplating the Umarex T4E HDS Shotgun .68 to play with here on the forum.
It is CO2 powered and there are  projectiles and molds for this that I believe would be allowed here.

 I would love to go all Elmer Fudd on a wabbit in da gawden wifth that for swure.

[Rob M]

Jack , I looked at the diagram again , and im likely missing something.. There are 2 major moving parts, the large spool or piston up front , thats releasing air into the barrel. The smaller part at the rear that is the firing pin , when that is struck , whats causing the large " spool " to retract rearward ? I assumed it was a pressure imbalance where the air at the rear is vented from the gun,, Perhaps its not

[jackssmirkingrevenge]

I tried to explain it in the video:



This is the crucial part:



The piston is slightly unbalanced and naturally wants to retract, but it's the locking balls (yellow) keeping it in place.

When the flare gun firing pin hits the purple pin, the locking balls are no longer supported and there is nothing preventing the piston from moving, so it can retract and dump the chamber.

[JuryRigger]

W-O-O-F!!!....   
72 FPE from that little package-awesome work!... 
Jesse

[Rob M]

I tried to explain it in the video:



This is the crucial part:



The piston is slightly unbalanced and naturally wants to retract, but it's the locking balls (yellow) keeping it in place.

When the flare gun firing pin hits the purple pin, the locking balls are no longer supported and there is nothing preventing the piston from moving, so it can retract and dump the chamber.

THANK YOU here i was not seeing the diameter change near the balls.  I fully understand now. The ball mechanism reminds me of a pull stud retainer  of a cnc machinign center.Brilliant

[Spacebus]

Are you going to try different types of Ammo? I assume there is no easy way to make it.