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Author Topic: PCP airgun design  (Read 495 times)

Offline Mintie

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Re: PCP airgun design
« Reply #15 on: December 29, 2018, 10:30:08 PM »
Either way there still needs to be a volume of air between the one way bottle valve (if it has one) and the valve face which would be considered the "plenum". The general rule is that you will need 1cc of volume in that space for each fpe you are wanting to get.
Lots of airguns

Offline Pauly5

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Re: PCP airgun design
« Reply #16 on: December 30, 2018, 09:15:30 AM »
Well I had that wrong didn't I, sorryabout that Ian.b.
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Offline mojo

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Re: PCP airgun design
« Reply #17 on: January 02, 2019, 11:53:25 PM »
Hi Ian, welcome to the forum.
There are a few of us on this forum who both build and tune/modify pcp's who could give advice.
First off what caliber/fpe are you intending on building ? If big bore are you intending on firing pellets or bullets.
I ask this as the build criteria can differ a lot depending on the intended end use. Danny.

Offline Ian.b

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Re: PCP airgun design
« Reply #18 on: January 03, 2019, 02:43:28 AM »
Hi, thanks for the welcome.  I think it will need to be a .25 as I dont have access to any firearm barrels, but I would like to build it with the possibility to increase the caliber at a later date, so maybe make the valve etc large enough to cope with increasing the bore.

It will have no purpose just for the fun of building and shooting it.

Offline Dvlnme

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Re: PCP airgun design
« Reply #19 on: January 03, 2019, 08:27:06 AM »
thats a perfectly good reason to want build a gun mate,many mine been built for exactly the same reason,
 just coz I could eh!!.
 cheers mike

Offline mojo

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Re: PCP airgun design
« Reply #20 on: January 03, 2019, 09:10:13 PM »
Ok, so if you are going to run a plenum I would suggest a minimum of .8cc per fpe for pallets and minimum 1.2cc per fpe for bullets. Attached I hope if it has worked is a photo one of my fairly early .50 caliber pistola made in house using a 13cu inch hpa bottle. This gun will give 5 fairly even shots delivering approximately 140 fpe. It fires direct from the bottle as I opened up the original 5/8" thread to about 20mm and threaded the valve block straight in. I needed to open up the thread so as to accommodate both the firing valve and a full capillary. If you can manage the same would negate the need for a plenum as the standard 5/8 neck can be quite restrictive on air flow even with any guts removed. I have also built plenty of guns using standard grade 6060 t5 alloy in 32mmx3mm filling up to 3600psi. I have made a tap to thread this material for the end plug however pinning will work also if you do not have the gear to do this.

Offline Ian.b

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Re: PCP airgun design
« Reply #21 on: January 04, 2019, 01:53:09 PM »
Thanks for the reply, you say you opened up the bottle neck for the valve and full capiliary? What is a capilary?  How do you manage to refill the bottle, do you have a fill port on the gun or is e capilary a small tube from the fillport into the bottle?

I will look into 6060 t5, havent heard of it 6061, 6063 and 6082 seem very similar in their make up/strength and 2024 t3 and 7075 t6 dont seem readily avaikable, at least not in 50cm orders.

Thats a smart little pistol, how do you achieve the power figures with such short barrel, is it the weight and diameter of the bullets?I

One last thing, when building your first gun how do you select the correct springs for the valve and hammer and are they readily available? Have you ever wound your own using piano wire?

Thanks.
« Last Edit: January 04, 2019, 01:57:32 PM by Ian.b »

Offline mojo

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Re: PCP airgun design
« Reply #22 on: January 04, 2019, 10:04:38 PM »
Typing error, there is a "fill" capillary down one side of the valve block which is fed from a foster type nipple which is visible on the side of the gun.
As for hammer springs there are a number of different factors which will determine how much air is released and how it is released with each strike of the hammer. These include but are not limited to.
Air supply pressure to the valve.
Valve seat diameter.
Valve seat angle.
Valve return spring energy.
Allowable maximum valve throw/stroke.
Port sizes and any constructions on air flow.
Hammer weight.
Hammer throw/stroke.
Hammer spring energy(guide a complex subject).
Resistance including slug weight and friction in bore.
I had no assistance or any idea how pcp's are constructed when I built my first pcp, however I fluked it and got it right first off.
As said there are a number of people on this sight who can give advice on build options, I am happy to assist in the hope that you build a safe and operational gun.
Again if you can give us an idea of what you would like as an end result both myself and others may be able to assist with advice on design and build.

Offline Ian.b

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Re: PCP airgun design
« Reply #23 on: January 05, 2019, 12:38:08 PM »
Safe gun is a necessity, and operational gun hopefully 😊, initially my hope was to build a bigbore, but that maybe out of the question due to the cost of a barrel, unless I tried to make a diy one with a rifling button off ebay as I am not try to make something super accurate, I will watch a few youtube vids.

It will need to be something like a large pistol with an AR style stock using a Crossman trigger frame as I havent a clue on making wood stocks.  Saw a few ideas for power adjustments I would like to try to use one s it can be turned down, I liked the simple ideal of a bolt through the transfer port.

Mojo - did you use a barrel from a rifle? If you dont mind saying approx what size transfer port and valve throat did you go with? It seems PEEK is ideal for the poppet, but will delrin/nylon/acetal be ok as unless I can order a sample minimum order I have found is 1/2 metre. Will a turned down high tensile bolt work for the stem or is there something better?
« Last Edit: January 05, 2019, 12:43:14 PM by Ian.b »

Offline mojo

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Re: PCP airgun design
« Reply #24 on: January 06, 2019, 09:57:08 AM »
The volume of air on board the .50 pistol is a big contributing factor to the total energy achieved from a 7" barrel.
The barrel was an off cut from Gunworks in Christchurch.
Most of the big bore pcp's I have built on recycled rifle barrels, some I have had the barrel pulled specifically for the build.
One of the challenges with using rifle barrels is the rifling pitch generally being too short/fast as in most cases we fire comparably short light bullets which tend to be over stabilized in a barrel intended for longer heavier bullets. This combination can then make it challenging to find a bullet which will group well. Second hand barrels are available for little cost, try your local gun Smith, .308 is a nice useable size with plenty of bullet mould options.
If you go big bore will you be casting/using cast ballets, or would you build for off the shelf .30-.45 slugs. Barrels intended for diablo type or similar slugs are available, wingman may be able to hook you up with a good one if you PM him.


Offline Ian.b

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Re: PCP airgun design
« Reply #25 on: January 06, 2019, 01:35:24 PM »
Thanks for info, I will look into pellets/slugs etc.  I bet that pistol you built makes an almighty noise!