Shotgun prototype

[pactin9]

To begin, my project was about halfway done before I ran the specs through GGDT (which is giving me numbers that I am not satisfied with). I wanted to create a spud gun that could be maneuverable (aka I don't want to lug around a 3ft barrel) and could make at least 6 consecutive shots before refilling.

For those reasons I chose the hammer valve as the choice of air expulsion. I took on an over under design with the compressed air chamber below and a hammer-T valve to vent to the barrel above.

The current specs are:
-24" barrel with a 1" bore
-21.5" chamber with 1" ID
-breech load at 6" after valve (initial position)

The valve:
-3/8" brass T
-unknown heavy spring
-3/8" valve seat
-about 0.75" valve travel

Projectiles:
-BB's 6mm and the little copper ones which I have more of.
-Paper shell

My questions are:
-My sched.40 PVC is rated at 450psi, would it be okay to fill to ~400psi with that length of chamber? I don't intend to fill it past 300, but I'd like to know I have some headroom.

-How can I get more power/velocity out of my projectiles without increasing chamber volume? For some reason GGDT says that increasing pressure twofold - from 100 to 200, that is - would hurt the performance.

-When I increase hammer force for the valve, it increases the power/velocity. Why is that?

-Would decreasing the barrel bore to 0.5" increase the velocity?

Sorry for all the questions, I'd like to know that I'm following proper spudding technique.

[metalmeltr]

I personally would fill pipe rated to 450 to 460 but other will advise against that you should be fine at 400. Increasing hammer force opens the valve faster and it opens it more so you have better air flow, this will probably be the easiest way to increase performance. If you can make your valve itself larger you will also increase performance. Test bore diameters in GGDT.

[jhalek90]

1in bore and a 6mm projectile??


this makes no sense.

[JDP12]

Shotgun jhalek.... as in shot loads...

[pactin9]

Metalmeltr: Ah that makes much more sense now I do have a self cocking hammer mechanism in the works so the opening force and speed shouldn't be a problem. In terms of making my valve larger, I might try to bore it out a bit with my burr set but it's a bit risky as I can't gauge the walls accurately.

Jhalek: perhaps I should've made myself clearer. The 6mm BB's (more than 10 at a time) will be loaded in a paper shell with a low mass sabot.

I wil try and post pictures tonight.

[al-xg]

You might find the need to make a choke, I've had very large spread from my shot shell attempts.

I did make a removable choke (tube split along the length, taped then inserted into a tube of the size above. More tape makes for a tighter choke, a ghetto version of this), but only ever tried it out on my rifled barrel which probably negates its effect.

[metalmeltr]

If possible could we see some pictures of the gun?

[pactin9]

al-xg: Thanks for the heads up. I thought about making a choke as well by inserting a smaller OD'd pipe into the end of the barrel (after smoothing out the edge of course). But currently concerned with my power; I fear my need for total aesthetics has led to the potential weakness of my gun

And as promised, some pictures:
Valve stem


Barrel and Chamber


Hammer valve

[metalmeltr]

That valve is nothing but flow restriction. A 1" sprinkler valve would be ideal. Or you could build a 1" piston valve.

[D_Hall]

-My sched.40 PVC is rated at 450psi, would it be okay to fill to ~400psi with that length of chamber? I don't intend to fill it past 300, but I'd like to know I have some headroom.

My personal opinion is that PVC isn't suited for pre-charged pneumatics, PERIOD. I realize my opinion is in the minority on that topic though so I'll drop it (I only bring it up because you asked).

-How can I get more power/velocity out of my projectiles without increasing chamber volume? For some reason GGDT says that increasing pressure twofold - from 100 to 200, that is - would hurt the performance.

Easiest way is.... a longer barrel, but you already said you don't want to do that.

As for the decrease in performance with a higher pressure: With just that to go on it's hard to say for certain, but I suspect the answer is that your hammer isn't strong enough to really open the valve against 200 psi pressure.

-When I increase hammer force for the valve, it increases the power/velocity. Why is that?

Because the valve opens quicker and stays open longer. Simple, really.

-Would decreasing the barrel bore to 0.5" increase the velocity?

Only if you decreased your projectile mass too.

[metalmeltr]

Whoops sprinker valve wont work in your operating pressure range-homemede piston could though.

[al-xg]

But then probably loose the multi-shot (and the power tends to balance out as the pressure drops with the valve staying open longer) but a hammer valve piloting a QEV or modified sprinkler could work.

I've just realised some of your initial questions weren't answered.
The length of the chamber shouldn't affect the pressure rating, the diameter/wall thickness material will though.
I'm not familiar with types of PVC so I can really comment on max pressures.
Maybe the higher pressure actually prevents the valve from opening properly, a stronger spring/ heavier hammer might be needed.
A heavier hammer will keep the valve open for longer (harder to slow down and then harder to acceleration).
GGDT sould be able to give an idea of the velocity change with a smaller barrel, although I would expect a smaller diameter barrel would work better in this case just because of the low flow and low volume of air released.

[metalmeltr]

Have you considered regulated HPA or CO2?

[pactin9]

But then probably loose the multi-shot (and the power tends to balance out as the pressure drops with the valve staying open longer) but a hammer valve piloting a QEV or modified sprinkler could work.

I would be glad to convert it to a piston valve if it were possible to make more than ~5 shots before refilling. The inspiration for this gun came from Patto's 6mm rifle which used a hammer valve as well but had a significantly smaller barrel than mine (6mm) at 100 psi. If I could salvage my project by going that route then I would.

[al-xg]

Oh I've just realised some of your initial questions weren't answered.

And am obviously partially blind, I hadn't seen the blind two previous posts.


I'd swap the chamber for a length of galvanised steel pipe or something and go for higher pressures.
Using a hammer valve to pilot would be a way of dumping more air with better flow and still have the multi-shot capacity. Basically the larger the Chamber to pilot volume ratio, the more air is dumped per shot (the valve stays open until pressure in pilot volume and chamber are the same, and pressure will drop slower in the chamber due to the larger volume).