The Craftmans Shop > Backyard Ballistics

Building a PCP airgun.

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NeoTech:

--- Quote from: shipto on January 17, 2012, 05:11:38 PM ---Nice job on the fly cutting but I am a little confused about your picture.
If I am looking at it correctly you pull the cocking bolt back and it opens up for the pellet at the same time. I am assuming your not going to have a magazine as there does not seem to be room, so each pellet will have to be inserted by hand.
one problem I see is that the spring will make it neccesary to hold back the cocking bolt while you insert the pellet this would require three hands or holding the gun with your knees.
The other problem is that i dont see anything other than the lighter spring holding the pellet probe in place, this would mean that some of the force would be lost and could make the power erratic.
Please feel free to correct me. I look forward to following your build, I have "the modern pneumatic airgun" by H.M. Buckley and did make a gun a while back but lacked the funds to do it properly with the correct materials so it was far from ideal.

--- End quote ---


Correct its a singleshot, handloaded version, due to the fact that i go with the bullpup design, meaning that the block will end up in the shouldrest i think i would be able to cock it and hold it open with one hand.. like onehanded mantle move on a glock..  It depends a bit on the size of your hands of course. =)

The pellet probe issue, well yes.. I thought a spring with enough force would do the trick, but i have got it pointed out several times now that it doesnt so i need to rethink that. The thing is that the dimensions of the block is so small that i cant really take off more material without comprimising the integrity of it. Maybe i could do some "pull out before pullback" and make it somewhat springloaded. I dislike those "push forward push down to lock" solutions, mostly cause they slow me down.

I have used several pcp air rifles over they years, and the best have been the dual pullbacks with seperate for loadpin and seperate for cocking. And of course the force lever ones, like the new FX 400'd.

Material, well so far im in 250gbp or so. i think i dont have to get more other than maybe more alu blocks if i screw the first one up. ;)
For stock i consider trying to make "Micarta" which is sandwiched epoxy drown cloth. So need to hit a thrift store and grab all the jeans i can  find! ;D

Brass_Machine:
One of the things I want to do is build an air rifle. When I move of course, because of the laws here. So I will be following this build with great interest.

I am curious as to how you are going to design this? What did you do for research? Where are you getting the info? I ask, because I would like to learn as well.

Good luck!

The fly cutting looks good!

Eric

NeoTech:

--- Quote from: Brass_Machine on January 17, 2012, 07:38:02 PM ---One of the things I want to do is build an air rifle. When I move of course, because of the laws here. So I will be following this build with great interest.
I am curious as to how you are going to design this? What did you do for research? Where are you getting the info? I ask, because I would like to learn as well.
Good luck!
The fly cutting looks good!
Eric

--- End quote ---

Well i'm gonna try to update this thread regular, my blog will be anyways.

Well designing.. I did what every blackbelt in google-fu would of done.. i Googled images of existing rifles, i looked for exploded view schemas on many different manufacturers rifles and i concluded what was the basic parts of a rifle and what was "fluff".. i can see reason for fluff in commercial parts.. But in my case i'm happy if it shoots. ;)

Next was looking up materials that could whitstand pressure. I went with aluminium tubes, steel tubes and different other alloys. I ended up with hydraulic tubing. its a 5mm walled steel tube
that can take around 6000 bar burst pressure, and are rated for around 2000 bar.. I guess it will not blow up in my face.. ;)
I did find different aluminium tubing, but aluminium is rather soft and i wouldnt wanna risk it. For fittings, valve and refill i will go with brass. Its tested througout the industry on higpressure vessels and couplings.. if it works for them it works for me.. 0.8-1mm pitch thread and a bunch of teflon o-rings and some dow-33 grease will probably handle the pressure well.

Another reason for brass fittings is that the air block is aluminium and steel + alu... not a good mix overtime i hear.. =)

The mechanics of a gun then.. Well i have been tinkering with devices that shoot stuff, launch stuff and blow up stuff on a regular basis for a couple of years.. I just went into solidworks and simulate the mechanics over and over until i find something that dont bind, break or blow up on me. The most basic is though, force drives hammer forward opening pin valve, force in airtube closes airvalve, letting small amounts of air out.. air valve is assisted by spring at lower pressures. Escaped air passes through a canal into barrel forcing pellet out of barrel. After that we will see. ;D

shipto:
ok first things first. I suggest you need to have the pellet probe locked in place during firing in one way or another, I personally like the push and lock down mechanism but whatever suits you. I would get rid of the light spring behind the pellet probe so that once the trigger has got hold of the hammer it will be free and easy to load the pellet.
weight is another issue to think about, H.M. Buckley suggests using aluminium tube for the reciever and having used the very same tube you are thinking of using I suggest the same. unless you are only thinking about using it for plinking in the garden you will rue the extra weight after a while and concidering you will probably only fill to 200 to 250 bar ally will be well up to it.
I would love to talk in real time to you about this topic if you have skype or something.

airstream:
Personally i would not use alloy tubing full stop for a air cylinder, stick with steel but 5mm wall is a bit over the top, unless the outside diameter is about 50mm. 2.5mm wall is ideal if using around a standard size  30mm od tube, its not to heavy and gives good air capacity. just make sure you hydro statically test the tube to a third above your working pressure to make sure your end plugs are safe.

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