The quest for a deer airgun

[BoyntonStu]

Alright, here is the video that demonstrates what I stated in the above post:

<a href="http://www.youtube.com/v/hBaNXMJHIV8&amp;hl=en&amp;fs=1" target="_blank" class="new_win">http://www.youtube.com/v/hBaNXMJHIV8&amp;hl=en&amp;fs=1</a>

You proved your clams.

This may be the most sensitive piston valve at 10 psi.

I have seen claims of 30 psi but AFAICR not lower.

I want one too!

Please list the fittings.

BoyntonStu

[Forum Admin]

I've taken some time to dust of the deer airgun project. I used a 26" half inch PVC barrel, firing a 278 grain roundball at 400 psi. The result is 413 fps, which translates to 105 fpe! At the same pressure, using a 948 grain slug in a 30" barrel, I get 281 fps and 165 fpe. That would leave a big hole in a deer, but I really need something that shoots over 350 fps to give a more crossbow like trajectory. I switched from a soft washer to a hard sink faucet washer as the sealing face. It has improved valve response time and withstands higher pressure much better than the old ones. I also added a floating o ring to the bondo piston, much like Stu did with his. The bondo wore down and performance began to degrade after a couple hundred shots. The o ring should last much longer and is cheap and easy to replace. Thanks for the most excellent idea of using a pipe cutter to outline the o ring groove! Using this setup, the valve will discharge on only 6 pumps, as opposed to the old 12! I'm guessing that is around 4 to 6 psi. This also brought the valve efficiency from 25% to 34%, a very good jump upward. The next thing I'll try is a oak piston and see if more performance can be had from a lighter material. I say that because the bondo piston succumbed to the shock after having the o ring groove cut into it after 2 max pressure shots. It broke in half. 
The good news is I have broken my old sprinkler valve record by 8 fps and 5 fpe, using a barrel that is 10" shorter with the .58 ammo.

[BoyntonStu]

I've taken some time to dust of the deer airgun project. I used a 26" half inch PVC barrel, firing a 278 grain roundball at 400 psi. The result is 413 fps, which translates to 105 fpe! At the same pressure, using a 948 grain slug in a 30" barrel, I get 281 fps and 165 fpe. That would leave a big hole in a deer, but I really need something that shoots over 350 fps to give a more crossbow like trajectory. I switched from a soft washer to a hard sink faucet washer as the sealing face. It has improved valve response time and withstands higher pressure much better than the old ones. I also added a floating o ring to the bondo piston, much like Stu did with his. The bondo wore down and performance began to degrade after a couple hundred shots. The o ring should last much longer and is cheap and easy to replace. Thanks for the most excellent idea of using a pipe cutter to outline the o ring groove! Using this setup, the valve will discharge on only 6 pumps, as opposed to the old 12! I'm guessing that is around 4 to 6 psi. This also brought the valve efficiency from 25% to 34%, a very good jump upward. The next thing I'll try is a oak piston and see if more performance can be had from a lighter material. I say that because the bondo piston succumbed to the shock after having the o ring groove cut into it after 2 max pressure shots. It broke in half. 
The good news is I have broken my old sprinkler valve record by 8 fps and 5 fpe, using a barrel that is 10" shorter with the .58 ammo.

Good work.

I eagerly await your oak piston results.

BoyntonStu

[Forum Admin]

I got my oak piston finished last night and did a bit of chronographing this morning. It fires the 948 grain slug at exactly 280 fps for a remarkable 165 fpe. That is at 400 psi. At 150 psi, it shoots a marble at 471 fps. I have yet to shoot a marble at 400 psi, it just might come close to the sound barrier. 
I've ran into a slight problem with this piston. For one, my shock pump wont seat it, I have to use my compressor to get it started. The other problem is that it wont fire by depressing the schrader valve. I'll eventually have to build a new one with tighter tolerances to fix the above problems. On the other hand, I'm thinking that quick steel would make a dandy piston as well, much more durable than bondo, but probably twice the mass of the oak piston.
At any rate, the oak piston produces as much power as the bondo, even when it isnt built to perfect tolerances. I guess this demonstrates the relationship between piston mass and end power. Could you imagine what a piston made of balsa could do...

[BoyntonStu]

I got my oak piston finished last night and did a bit of chronographing this morning. It fires the 948 grain slug at exactly 280 fps for a remarkable 165 fpe. That is at 400 psi. At 150 psi, it shoots a marble at 471 fps. I have yet to shoot a marble at 400 psi, it just might come close to the sound barrier. 
I've ran into a slight problem with this piston. For one, my shock pump wont seat it, I have to use my compressor to get it started. The other problem is that it wont fire by depressing the schrader valve. I'll eventually have to build a new one with tighter tolerances to fix the above problems. On the other hand, I'm thinking that quick steel would make a dandy piston as well, much more durable than bondo, but probably twice the mass of the oak piston.
At any rate, the oak piston produces as much power as the bondo, even when it isnt built to perfect tolerances. I guess this demonstrates the relationship between piston mass and end power. Could you imagine what a piston made of balsa could do...

Those are good numbers.

Very encouraging.

Add some Teflon tape or even dental floss inside the grooves to tighten the seal by making the O rings sit a bit higher.

What size barrel did you use?


A valve between the shock pump and the Schrader will allow you to bump the piston closed.

I believe that it would be a better solution to sealing than a return spring.

Stu

[Forum Admin]

I'll try that teflon tape and see if I cant get a better seal. Once it seals better, the shock pump should be able to seat the piston. There's just too much blowby right now. I too prefer to stay away from the return springs. In theory, they seem like they would slow the valve down a bit and rob you of power. I used a 3/4" type L barrel for the slug and 1/2" schedule 40 pvc barrel for the marble.

[Forum Admin]

I havent taken the piston out, but I took dozens of shots yesterday to test its durability. So far, no problems at all, which is great considering it only took a few minutes to make. I've got to find my camera charger so I can show some damage pictures and videos, as they are quite impressive. It will shoot a load of 4 slingshot bearings (.375 diameter) with amazing force. I need to build a choke to tighten the pattern. At 12 yards, it shoots all 4 into a 16" circle. I shot at a closed 2 liter bottle from 20 yards, but only one hit the bottle (a complete passthrough) at that range. The others tore up the ground all around it. Each bearing has about 25 fpe, which is comparable to the energy of a good adult airgun.
Once I get the piston tuned, it will be time to build a trigger and stock for the airgun, both of which I already have roughed out. I might actually get this thing done in time to hunt deer with it.