.25 Air rifle, Breech loaded, rifled barrel

[cane toad]

Hi all,

As a carry on from our previous attempt http://www.spudfiles.com/forums/first-r ... 22714.html we decided to build another rifle, hopefully improved.

The priorities of this gun were accuracy over distance first, then power.

It may help a few other aussie readers to know that this is Australian made (from Chinese parts of course).

The new rifle has the following specs/parts:
Compressor filled
(empty) Soda stream bottle as chamber
QEV valve from ebay http://www.ebay.com.au/itm/G3-8-Pipe-Pn ... 459wt_1139
Blowgun as trigger
Brass fittings as needed
steel barrel (aluminium barrel shroud)
Aluminium stock/frame
Small piece of compressor hose for breech
Fires .25 Air rifle pellets

I should have taken some photos before I lazily sprayed the whole thing black.

A few thoughts:
We kind of made this thing up as we went, sort of finished it then decided to change/improve something, which is why in the end I've cable tied alot of parts on (easy to remove/replace). The only parts riveted are the frame to the barrel, so the sights don't move if the rifle gets bumped.

The barrel we used is .25 steel pipe, which we rifled using a ground down piece of tool steel. .25 air rifle pellets breech load perfectly with no gap (they won't drop into the barrel).

We drilled out all the fittings as much as safely possible to get a bit more air flow/power.

We tapped in the tank fill valve on the brass elbow above the blowgun, and tapped a pressure gauge in the end of the chamber to keep an eye for leaks etc.

We got hold of an old fridge compressor and have played around with it a bit. The most we've taken it to is approx 160psi, next step is a bigger gauge and higher pressure testing.

A bipod is on the way and will be fitted.

So far we've only tested the accuracy to 80 feet, and we can get approx 2" groupings. Part of that is due to human error and the fact I've only got a 4x32 scope. I might vice mount it and get a real accuracy reading (and buy a better/bigger scope).

Lastly I have to mention this was a two man job, my workmate deserves alot of the credit.

I've probably forgotten to include a lot of information, if anyone has comments/questions/criticisms I'm all ears. I can also provide more photos but there's no interesting home made pistons or anything.

Thanks for reading.
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EDIT: I've added some more photos, self explanatory.

[jackssmirkingrevenge]

Much better than your previous effort! Please tell me there are some holes in the muzzle end to bleed air into the shroud

600 psi gauges can be had for not so many of your finest Australian dollars, crank it up to 450 psi or so (about the most that blowgun will take without leaking) and you will be fapping at the results of the power difference

The chamber is far too big for a single shot though, consider purchasing a slide valve and adding a secondary chamber to make it a repeater.

[POLAND_SPUD]

OMG... that's pretty amazing

I really like how you bent metal to form the stock, pistol grip and the rails (ohh btw what did you use for that and where did you buy it?) ->> look ppl that how you do stuff

[jackssmirkingrevenge]

I really like how you bent metal to form the stock, pistol grip and the rails

Reminds me of an early model Sten

[cane toad]

Wow that was quick. Thanks for the suggestions, JSR. Shouldn't you be in bed though???

I have not drilled holes in the end of the barrel, that's a good idea, should have done that before I rifled it.

Just ordered a couple of those 600psi gauges, thanks for the link, they'll come in handy.

Ahh I was wondering what the magic number was for the blowgun pressure. 450 it is then.

Yes I realise the chamber is wayyy too big, just had that bottle lying around and I liked the look of it. Also it was a bit of a f-you to my old gun that had to have a tiny chamber so I could pump it up with the shock pump.

I have thought about turning this into a repeater the way you described, but I'm so sick of taking this thing apart I think I'll save that for the next attempt.

Thanks again for the suggestions.

---------

Hi Poland Spud, and thanks.

Just like the last one all I used for the stock/grip/frame was 20mm flat aluminium bar. It's easy to bend in a vice and pretty forgiving. I screwed all the other parts together first and laid it out, then every bend I made I just checked to make sure it would actually fit the gun.

The rail is just 10mm aluminium flat bar. I used a bench grinder to bevel the sides so the mounts would hold, and riveted it to the frame.

I hope that answers your question. Thanks again.

[Crna Legija]

you could put a small hole in the piston and a spring behind the piston, then if you tap the blow gun you should give you semi. Just like BTB didhere

looks great, get some damage pic once you up the pressure.

[jackssmirkingrevenge]

Thanks for the suggestions, JSR. Shouldn't you be in bed though???

At 9 in the morning? I should be working

Ahh I was wondering what the magic number was for the blowgun pressure. 450 it is then.

That was my experience with that type of blowgun.

I have thought about turning this into a repeater the way you described, but I'm so sick of taking this thing apart I think I'll save that for the next attempt.

Here's a (very!) rough concept drawing, the bottle becomes the stock attached to the slide valve and the firing chamber is made much smaller.

[cane toad]

Crna Legija - Thanks for the link, I'll check that out and keep it in mind for my next attempt.

JSR - Sorry, I have a bad habit of assuming everyone on the internet is in the USA unless specifically stated otherwise

As for the slide valve, I've never used one but my mate just received one he ordered by accident (how fortunate). I don't know what they do or how they work but I guess I'll have a play with it now.

PS thanks for all the good ideas, guys. I'll get the itch again eventually to build another one and I'll be incorporating some of these.

[Crna Legija]

As for the slide valve, I've never used one but my mate just received one he ordered by accident (how fortunate). I don't know what they do or how they work but I guess I'll have a play with it now.

the pilot is a slide valve in the gif.

[zach12]

I really like the frame/stock. looks clean. Is it just a long aluminum strip? I was planning on making one myself

Edit:

The rail is just 10mm aluminium flat bar.

Sorry I missed this comment.

[Gun Freak]

This thing is awesome... Really nice job

[Lockednloaded]

Looks great! Much better power then the last one I assume. The chamber is way overkill for a .25 barrel you could have one 1/4 the size without losing much power, and plus, itd be much easier t charge with a shock pump

[cane toad]

Crna Legija - Thanks for the diagram. Looks a bit above my pay grade. I'll look into it for the next one. Thanks again.

Zach12 - Thanks. The scope rail is 10mmx3mm alum, the frame/stock is 20mmx3mm alum, 10mm might be too flimsy for the frame.

Gun Freak - Thanks!

Lockednloaded - Thank you. Yes it has more power than the last one. You make an interesting point, I know the chamber is way too big (not a deal breaker for me since it was a scrap part ie free and I'm filling with a compressor) however when I tried a smaller chamber (using a length of hydraulic hose like my last gun) I seemed to lose a lot of power. I haven't done any further testing in that regard, ie what the optimal chamber size would be.

Which reminds me, I downloaded the GGDT program and I'm a bit lost. Could anybody answer the following (or point me to a newbie link)
* What is flow coefficient? How to I measure that?
* What is the seat diameter? Mine is a barrel sealing piston, is it the diameter of the sealing face (ie barrel i/d)?
*Piston mass - the only moving part in my valve is a rubber seal/bucket. it has next to no mass (weight). So do I put a very small number in here?
* Is vent diameter the gap around the piston that bleeds to fill up the chamber (in my case)? Is it measured in diameter because a piston with o-rings has a vent hole drilled in it? What if you have a piston like mine with no o-rings and air bleeds around the outside?

I hope those questions make some sense. I have tried searching, but if they've been asked 1000 times before please point me to the right threads.

Thanks in advance.

[saefroch]

Generally I just leave the flow coefficient as it is, as I understand, the default fits a normal piston valve.

The seat diameter, I think, is also called the port diameter. It is the diameter of the smallest area that the gases must flow through to reach the barrel from the valve. Sometimes it's barrel ID, but often not.

Measure the piston mass. You can guess, but I don't recommend it.

The vent diameter is the fully opened port diameter of the pilot valve. GGDT assumes that the equalizer hole is insignificant.

[cane toad]

saefroch - Thanks for that information, that makes a lot of sense. I wouldn't mind setting this gun up in GGDT so I can see what changing a variable does to the performance. Up until now I've had no theory, just practical experiments/tests.

Thanks again.