PCP queries.

[inonickname]

Well whether or not you know I'm planning on building a PCP rifle (for the soul purpose of plinking, if I hunt I use more powerful weapons)(when I get my lathe-hopefully sooner rather than later.). The barrel is .378 cal (extremely close to it, I was surprised when I measured it) aluminum and the chamber has a 3/4" O.D. I don't recall the I.D. at this second. Op. pressure will be 300-500 psi depending on if I make a single or two stage pump.

The valve will be a hammer valve, which leads to my first problem. It will be struck by a hammer of some kind with force imposed by a spring. This is the part I'm not sure on; use a compression spring and a sliding hammer or use an extension spring and a rotating kind of hammer, as you would see in a traditional firearm. Are there any opinions of this? I'm leaning towards the extension spring.. But I don't have much experience, so has anyone tried the more traditional hammer like this.


Anything I should know about using them?

As it's a short-mid range plinker, it probably won't be rifled (will be using foster slugs as it is) but seen as the barrel stock is aluminum it wouldn't require masses of force to rifle it (button rifling). The problem is a jig that can be turned by hand or machine using reduction threads, but cut at the correct angle. If I can figure out a jig to do that then it may as well be rifled. Again, suggestions welcome.

As it's a plinker, probably open sights and a laser.

And lastly, I'm not sure how to calculate a good barrel length. The chamber outsizes the barrel by a lot, but not all of the air available is used for each shot. Also, I want fair accuracy but lower weight. Assuming these factors, would you assume that 36" of .378 cal barrel is sufficient? Gah, I feel like I'm asking for a spoon (not really, just suggestions or advice..)

It's inspired a lot by the Girandoni repeating airgun

Any thoughts, advice etc? Thanks

[USGF]

inonickname, What machines do you have access to? The monoblock design you propose is pretty ambitious. Assuming a bench mill, I think the swivel hammer is the easier to set up. If you are working this job entirely on a lathe, I would go for a plunger type hammer. Look forward to seeing your results.

[inonickname]

inonickname, What machines do you have access to? The monoblock design you propose is pretty ambitious. Assuming a bench mill, I think the swivel hammer is the easier to set up. If you are working this job entirely on a lathe, I would go for a plunger type hammer. Look forward to seeing your results.

It's not my design, it's simply an example.

If I were to make something along those lines I'd make two blocks then permanently bond them together. I think I know what you mean by swivel hammer and plunger, but could you elaborate on them?

Tools would be basic tooling etc +lathe and mill most likely.

I have some early drawings done, mostly on the hammer and triggering mechanism. I'm making a small prototype hammer tomorrow to test.

[Gippeto]

Having had some problems with an inline hammer , it doesn't take much imagination to say that it would be easier to increase the mass of an external hammer should the need arise. There would be more room for heavier springs, and increased travel (especially) as well.

BTB has used external hammers on a couple of projects. If he doesn't chime in, you might consider pm'ing him.

As to a rifling machine, this one is pretty low tech. It's for "cut" rifling though.

[Hawkeye]

I would go with an inline "plunger" hammer. A drill bit extender slides perfectly in half inch copper and would make a perfect hammer if you polish it. It is substantially heavy and it likely wouldn't need a long travel to knock the valve open.

[far_cry]

if you want build a pcp air gun at least buy a rifled barrel ( 100 $ for something good in usa )

and here a photos show you exactly how this work. i don't know what plans do yo have . in this pictures you will see the mechanism of the airforce condor\talon

and if like what you said about lathe and milling machine ,thats will be no problem

[inonickname]

Thanks guys!

Farcry, those diagrams are quite good. So you cock the gun, laod the pellet and when the trigger is pulled the bolt slides back, sealing the barrel and striking the valve?

I think I have a basic improvement over that design, I'll get back to you guys.

[far_cry]

So you cock the gun, load the pellet and when the trigger is pulled the bolt slides back, sealing the barrel and striking the valve?

you miss something , when you cock the gun you must return the breech slide back to the valve stem ,like in the last pic
then you pull the trigger and the hammer strike the breech slide while she tight close
capish

[twizi]

here found this on the net its a bullpup pcp diagram


http://www.thefirearmblog.com/blog/2009 ... r-gun/[url][/url]


end here http://www.beemans.net/images/Operation ... 9k.gif[url]

hope that helps[/url]

[far_cry]

did you mean this

http://www.thefirearmblog.com/blog/2009 ... p-air-gun/

[jonnyboy]

I too think an inline hammer would work well. In my mind it would be easiest to work with as a start ie; travel length, spring size and force. Later you could then make it more traditional if you want.

[inonickname]

Thanks guys. I think I'll go with an inline configuration such as the condor, as that is obviously a working design. I'll use an old soda stream cylinder or a small fire extinguisher as a chamber.

Though the one thing I have to improve on over the condor is that I'll use a large firing reservoir (larger, longer barrel) and have a piston that blocks the main reservoir on firing, meaning I can use a valve with higher flow without letting unwanted air through.

Probably the largest problem is the spring. Any ideas on how I calculate the amount of pressure vs surface area of face vs hammer weight vs spring strength vs spring travel vs volume of air to dispense, if you know what I mean?

Basically how strong a spring to use for what pressure behind the face and hammer weight.

Thanks y'all!

[far_cry]

it's a bit hard. i have learn this stuff 5 years ago and i was good with
my home work (not high school .after) i solve some examples and i have no idea how was i solve this out any way the least you want is the spring data. in N\m, not the torque Nm

google this Hooke's law

edit
here you can see how to know the force you get from the pressure on some surface

[inonickname]

Wonderfully simple.

I'll just do some basic calculations concerning force behind the valve stem, strength of the spring and valve weight. That's not too hard.

For adjust ability I'll add a small sleeve around the barrel (threaded) and add a collar (eg. -nut) to adjust preload et cetera. Should take some of the pain out of getting the perfect spring.

Thanks Farcry- when the design gets closer to completion I'll run some numbers off to someone who can get this done right- while I'm quite fair at math I don't want to miscalc something, then have two fingers cut off by an overly strong spring during loading. Something that strikes me about the inline, if the sear fails and you're loading it then there will be trouble.

Thanks guys, I drew up some plans tonight. I'll upload them if I can get to a scanner.