Ammo feeding for a QEV/popoff cannon

[Xenophile]

Hello Spudfiles,
I've been working on a cannon that uses a Quick Exhaust Valve (QEV) as my main valve and a 200 psi safety popoff valve to give me a fully automatic rate of fire. I'm using a 3 position 2 way (3/2) valve as my trigger, and I have an old crossman barrel in .177 caliber (4.5 mm) attached to the "R" port on the QEV. I'm also using a neodymium magnet as a detent. I was using a T fitting as a gravity fed magazine, but unfortunately the 200 + psi air is enough to push the lightweight bbs to the top of the magazine without feeding them into the barrel. Do you have any advice on how to build a simple spring loaded magazine or some other ammunition feeding device?

I've had great success with 3/2 + QEV setups before, but I've used larger and heavier ammo with those (ex: steel slingshot ammo in .30 caliber). Any advice would be appreciated. I'm almost wondering if I should build a BBMG instead, like Burnt Latke's design, but I thought a QEV + popoff + 3/2 would be more efficient. Thanks!

[farcticox1]

I've had that happen before, I put a T near the end of the magazine tube and supplied it with a small amount of air from the barrel (another T)
Sort of like this

[Xenophile]

Clever design! I like the idea of using air from the barrel to push the ammo down in the magazine. Does it make reloading more difficult though? Currently, I'm waiting on some epoxy to cure on a "follower" piece that I will be adding to the inside of some ~5mm metal pipe to make a spring loaded magazine. I've cut a long "L" shape into the pipe for the follower to run down and compress springs. No idea if it will work, but tinkering is a huge component of this hobby. Thanks!

[farcticox1]

No idea if it will work, but tinkering is a huge component of this hobby. Thanks!

That's pretty much how I do it. I have added another T to that which makes it easier to load, I will do some videos some time, still settling in to a new house.

[Xenophile]

Thanks again, although I believe I've found a different solution.

This is a crude magazine I've cobbled together with some brass pipe, compression springs, epoxy and heat shrink tubing. The capacity is about 50 .177 bbs. There's a large "L" shaped groove I cut to allow the follower to push bbs into the barrel.

This shows the magazine in the "locked" position allowing me to load ammo into it.
It seems to work, though I have to manually hold the magazine to the cannon. Otherwise the air pressure is too great and the magazine pops off. The next step is to build a better mag well.

Oh, here is an overview of the gun if anyone is interested.

And these are the internals. The insulation foam is to muffle the sound of the popoff valve.

It may not look very pretty now, but with some wood filler and stain, I hope to have look a little nicer than the "random plumbing parts and pneumatic valves" current aesthetic.

[jackssmirkingrevenge]

Looking good!

Is the a bolt to interrupt the flow of air to the magazine or do you just rely on the stacked BBs?

[Xenophile]

There's no bolt currently, although I might mess around with adding one in the future. It seems to function fine without, though I'm probably negatively impacting performance. In the meantime, I'm still working on improving the magazine and its attachment to the gun. I'll keep you guys posted.

[farcticox1]

So it's firing one BB every shot ? I made a semi-auto airsoft with a bolt which blocks the ammo feed tube which works great, now that it's sorted

piston.jpg (11.86 KiB) Viewed 10061 times

[Xenophile]

It appears to only shoot one bb per shot, but with its high ROF from the popoff valve, it's a little hard to tell. I'll probably disconnect the current barrel/magazine assembly from the current gun, and attach it to my reliable QEV semi-auto gun just to verify that the current ammo feeding mechanism works.

Thanks for the bolt schematic. What did you make yours out of in case I want to try building my own?

[farcticox1]

Aluminium block I found, it was quire a lot of work on the drill press. Getting the right spring and piston was the tricky part.
viewtopic.php?f=29&t=27151&sid=2c135531 ... 15f147a9ca