Cannon #3

PCGUY · Posted: 09/03/2009 7:07 AM Post subject:

Cannon #3

14mm Automatic Air Rifle

SFDBGOEPHP14ABAR (or the 14mm automatic air rifle)

"Select Fire Delayed Blowback Gas Operated Electro-Pneumatic High-Pressure 14mm Automatic Bullpup Air Rifle"

Final Judges Score Chart
-Idea 12.1/20
-Use of materials 2.7/5
-Creativeness 5.2/10
-Quality of build 1.4/5
-Function 1.8/10

This cannon is part of the SpudFiles Innovative Contest. For details see here...
http://www.spudfiles.com/forums/spudfiles-innovative-design-contest-ju dging-t19233.html

--Details and pictures from cannon builder--

Originally this design was meant to be built with a lot of care and precision.
It was meant to use a custom made high efficiency Quick Exhaust Valve machined from EN-AW 2014 T6 aluminum.
It was meant to run on 850psi dry air and be able to send various projectiles over 218yards (200m) down range in rapid succession.
Oh and it was meant to look good too.

However a lot of things went wrong and this project couldn't be built as planed, in time.

But this competition focuses on the innovation in the world of spudguns, so I decided to make it anyway from parts I had lying around and that would be available to most spudders.
This ghetto version, runs on high-ish pressure instead, is less compact, powerful, accurate and efficient, it also isn't very pretty.
However the ghetto version still uses the same principles.

Ghetto specs:

Overall length: 130cm
Bore size: 14mm (.55 cal)
Barrel length: 110cm
Rifling twist rate: 1:8inch
Main valve: Slightly modified 3/4" QEV for better flow.
Chamber volume: 160cc
450psi regulated air from HPA bottle.
Pilot valve: Modified electro-pneumatic 3/2 NC 1/8" solenoid valve (to work as NO)
Safety, semi-auto and full automatic fire modes.
Pump action manual reloading.
Gas piston controlled delayed blow back for auto-loading.
Electro-mechanical automatic trigger system.
Detachable 10 round magazine.
Bullpup configuration.
Rail mount for optics .

This is how it operates step by step:

1- The HPA is connected, flows through the 3/2 distributor and pressurizes the system (pushing the QEV piston forward and making a seal against the barrel port).
2- A loaded magazine is inserted
3-The first round must be loaded, the front grip is pulled back. This does two things: the bolt lock is pushed open and the bolt is pushed back. As the bolt moves back, a round from the magazine is pushed up by the magazine spring.
4- The grip slides back into initial position. The bolt is spring loaded and also returns to initial position, pushing the projectile past the main valve port and into the barrel. As the bolt moves forward, the bolt lock clicks back into place.
5-The fire mode switch is moved from Safe to Semi-auto.
6-The trigger is pressed and the 3/2 valve moves to firing position, the pilot volume is vented, the piston flies back and air from the chamber is dumped into the barrel exerting pressure on the projectile and on the bolt.
7-The projectile accelerates down the barrel following the rifling grooves. The bolt on the other hand cannot move back thanks to the bolt lock.
8-The projectile passes in front of the gas piston port, the air pressurizes the cylinder and pushes the gas piston back. The gas piston acts on the bolt lock rod, and opens the bolt lock.
9- With the bolt lock open, the bolt can recoil back using the pressure remaining in the barrel. The projectile is nearly up to speed and acceleration starts to drop as it is about to leave the barrel.
When the bolt recoils back, the front grip is not affected.
10- The bolt hits the spring in the buttstock, and its backward motion is absorbed. A new round is pushed up from the magazine. By this time the first projectile has left the barrel and is traveling down range, stabilized by its rotational movement along the firing axis.
11- Pressure in the barrel has now left, the bolt and projectile move forward, the gas piston returns to initial position and the bolt lock clips into place. The new round is now loaded.
12- The trigger is released and the 3/2 valve moves into filling position, air flow through it and resets the piston filling the chamber with 450psi regulated air.

If the trigger is pressed again the process repeats from stage 6-11

If on stage 5 the switch is set to full auto (the alternative stages being called 5', 6', 7'... and so on), the trigger switch is no longer linked directly to the solenoid valve but is connected in series with another micro-switch.
This second micro-switch is activated by the bolt being in forward position.
This is the case following stage 5' so when the trigger is pressed on stage 6' the 3/2 valve fires.
Nothing changes until stage 9' when the bolt starts to recoil. As it moves back, it releases the micro-switch which breaks the circuit and allows the 3/2 valve to go back to filling position.
The chamber starts to repressurise. Once the projectile leaves the barrel and the bolt comes back loading a new round, it hits the micro-switch causing the 3/2 valve to fire once more and the process repeats until the trigger is released.
This also insures that if there is a jam and the bolt or round gets stuck the valve won't release the air.

The system is built so that the grip can act on the bolt and bolt lock rod, but the bolt and bolt lock can't act on the grip.
The gas piston can act on the bolt lock rod, but the bolt lock and grip do not act on the gas piston.

The delayed blow back means a lot less energy is wasted on the bolt and the shot is therefore more efficient.
This might not be crucial for smaller calibers but is a big advantage with larger bores.

A few problems I encountered building the ghetto version:
Lower operating pressure dramatically decreases the power and increases chamber filling time, thus reducing the rate of fire (with consistent full power shots).

The aluminum I used for the bolt lock isn't strong enough, after a few shots it bends back and doesn't open. Higher grade aluminum or steel should be used instead.

Some parts of the solenoid valve aren't designed to hold pressures that high and cause the valve to leak. I had to seal some parts together using epoxy glue and cast an epoxy block around it to reinforce it. The spring around the ferrous solenoid core had to be stretched and hardened as after multiple shots it would start to leak over 400psi.

I didn't have enough epoxy left and had to use hot melt glue and tape...
I had to rely on fridge compressors to fill my HPA bottles as I didn't get to finish my high pressure filling station setup and was limited to 650psi fills. That obliviously reduced the shot capacity compared to a 4500psi fill.
Knowing how far down the barrel to place the gas piston is quite tricky, I was lucky and it worked on the first try, I was expecting to have to experiment a lot more.

I still plan on making this design properly, I just don't have the time right now and still don't have the custom QEV I designed.
I haven't been able to do any nice videos, although I had a lot of good ideas to start off with, they will just have to wait for the final version.
I haven't even really had time to play around with this version, I fired only a few test shots, broke and bent a few parts, and didn't have time to improve.