Pellet gun

[Juggernaut12121]

Sorry for all the stupid questions, but I don't know who else to ask. I was looking to get an air rifle a while back for target shooting in my yard (longest line of sight is 75 yards) but I couldn't find an airgun that I liked that wasn't several hundred dollars. So I figured I would make my own
The only problem is I have no idea about the thermodynamics of the hammer valve plus some of the finer details of construction, and how to make an efficient and accurate PCP. I've found a guy in England selling some book all about how to make a PCP (The Modern Airgun by Howard Mark Buckley, I think it's called) which I think would help but I don't know how good the actual product is.
Finally, I've thought of making a pellet gun from a QEV but I don't know that the pressure rating of it would be able to compete with a PCP's, nor would the range, although it seems it would require a lot less machining and specialty parts (I have very limited access to a shop). Any and all suggestions and help are much appreciated!

[jackssmirkingrevenge]

Using modest pressure, a QEV rifle would be able to compete with a PCP, but if you want accuracy then you're going to need to source consistent projectiles, a rifled barrel would be essential unless you planned on making your own slugs that are capable of stable flight out of a smoothbore barrel.

[Juggernaut12121]

I was planning on finding some blank airgun barrel or perhaps a replacement one for better accuracy, along with some actual pellets (not made incompetently by me). And what would you consider modest pressure?

[jackssmirkingrevenge]

GGDT suggests that at just 300 psi with a fast efficient valve and a 4" by 1" chamber and 24" barrel you could rival the performance of a UK legal 0.22" PCP, though it would be a one-shot affair.

pelletgun300psi.png (24.74 KiB) Viewed 5905 times

[Juggernaut12121]

I'll have to play with ggdt some, 500fps isn't bad but it doesn't quite rival some commercial pcps and springers.
Dumb question, how do you find flow coefficient? I've got an idea but I don't know that's it's right

[jackssmirkingrevenge]

I'll have to play with ggdt some, 500fps isn't bad but it doesn't quite rival some commercial pcps and springers.

Unless you can access higher pressures, this is what you'll be limited to.

Dumb question, how do you find flow coefficient? I've got an idea but I don't know that's it's right

http://www.thehalls-in-bfe.com/GGDT/usage_Cv.html

[bravootome]

I would use a hammer valve with 40-60 bar ( witch you may achieve with a fridge compresor) min pressure and a rifled barrel, for the distance you're talking about....

As about the hammer valve "how to" built there are lots of informations on youtube with people modding thre's guns...
Myself i used a fire extinguisher witch already has the valve, and a gauge....

[Brian the brain]

Finally, I've thought of making a pellet gun from a QEV but I don't know that the pressure rating of it would be able to compete with a PCP's, nor would the range, although it seems it would require a lot less machining and specialty parts (I have very limited access to a shop).

A QEV launcher can be made to rival, if not surpass a commercial PCP when it comes to power and range.
A large bore will give you a lot of energy easily.

Shot count from a single charge however is a different story.
QEV's operate at much lower pressures and get their power from opening speed and high flow.

So compared to a commercial hammervalve it will waste more air while the valve cannot directly handle the extreme pressures of HPA.Without a HPA bottle and a regulator to bring down the firing pressure, you are limited to what you can store onboard...wich means more capacity will lead to bulk..

Still, with one of my guns..at 45 bars I shot 7 marbles through a fridge on a single charge.The gun has a traditional rifle shape.
I went overboard on the look , stock and mechanics of the gun, but it would have worked just as well if it were made from standard fittings. And at low cost.


Accuracy depends heavily on the barrel and ammo you use and to some degree, the ergonomics of your design.

[Juggernaut12121]

Sorry for taking so long to respond, I've had a while to play with GGDT and some ideas.
I was going to make my own QEV to handle higher pressures, along with this I plan to operate it off of a 20 oz CO2 cylinder (I've got at least three lying around along with an ASA fill adapter, to adapt the threads over to 1/8'' NPT). As I've said I've went about comparing the two valve models on GGDT and it seems that a QEV rifle will rival a hammer valve pretty well. So far my current configuration yields results of almost 900 fps (I want to get a high velocity for power but still stay under the sound barrier so that the pellets won't destabilize after being fired).

Some GGDT specs

Extending the chamber to 6" knocks the velocity past that 900 fps mark , which rivals the closest PCP rifle (Benjamin Discovery). Although I'm unsure of how long my CO2 tank will last (I realize there's probably a pretty simple way of figuring this out, but I haven't done this yet).
I'd like to try and stick with a QEV, correct me if I'm wrong but I feel they'd give more consistent power/shots since the chamber is being filled to a consistent pressure. I was also wondering, how consistent are paintball tank regulators for air? And could I use one for regulating the CO2? I was going to use a pop off valve for the trigger (pilot valve), but being a pop off valve the gun would fire if over pressurized.
One final concern is the breech access, so far I only have the idea of a piece of the barrel cut out near the base for the pellet to be loaded into, and then two o-ring grooves to be cut into either side of the slot and a piece of tube that slides over the breech and seals with the o-rings. I don't know how efficient this will be, any thoughts?
If it helps, I could upload some of my drawings to show where I'm at right now as far as design goes. Thanks for all the feedback

[Gippeto]

Reality check time.

If you get 900 fps on co2, you'll be the first to do so. Some have claimed 800fps, but their claims remain unsubstantiated as far as I know. Have done mid 700's so figure the 800 claims are at least possible.

Pellets do not destabilize by shooting them through the sound barrier. They destabilize when they pass BACK through the sound barrier....keep them fast all the way to target and they WILL group. Have seen it on a Condor shooting .177 @ 1400fps.

Paintball regs are pretty good. Not sure how they would handle co2, but construction is basically the same as the Palmer Female stabilizer (paintball co2 reg)...might work. Would likely have to lower the set point though to ensure you're metering gas and not liquid.

The Condor (and other Air Force air rifles) use a sliding breech cover, as do some of the Korean big bore air rifles (Fire 201). Sometimes "absolute" efficiency takes a back seat to practicality...don't lose any sleep over it.

Drawings and diagrams are always helpful to convey an idea...if you have them, post them. Can't hurt and might help.

Regards

[Juggernaut12121]

Reality check time.

If you get 900 fps on co2, you'll be the first to do so. Some have claimed 800fps, but their claims remain unsubstantiated as far as I know. Have done mid 700's so figure the 800 claims are at least possible.

Dream crusher
I thought it was a bit odd that CO2 yielded such a high velocity but I guess I like to fantasize. Would air power be a better alternative? Or would GGDT's estimates still be pretty inflated? I was going to try and make due with CO2 since I have 3 or 4 tanks already laying around, that and air tanks are a little pricier.

I knew that pellets had destabilized only after going back to the sound barrier but I hadn't thought of keeping them supersonic all the way to the target, I think it would be easier with such a home made gun to keep it sub sonic than go super sonic.

I don't know how much it will help but here's a cruddy drawing.
The chamber, barrel support, and parts of the stock and handle are made from iron plumbing fittings, the QEV will be machined from a steel block to handle the pressure.

[Gippeto]

Yep...'tis I.

Using air will allow you to make more power. No reason you cannot use co2 if you want to, just lower your expectations a little.

Have you considered using a slide check in place of the relief valve trigger? Will shut the flow off from the supply while venting the qev at the same time. Just a thought.

If you've not purchased your coil line etc yet, can get one with a slide check so you don't have to buy it separately.

http://www.ebay.ca/itm/New-Paintball-Co ... 5b0e1db0e8

[Juggernaut12121]

Would I be wrong in assuming that a 20oz CO2 bottle would give me more shots than a 48cui 3000psi air tank? I figure I'll probably stick with CO2 at the moment and upgrade later if I see it fitting.
I was thinking of a different trigger/vent system than the pop off valve, especially after playing with one today. My only concern with the slide check is how accuracy will play into it. I can't say I've ever seen a slide check on a paintball rig though, I'll have to keep it in mind, it should be better overall than my idea.

[Gippeto]

Don't over think this, you're building an IMPROVISED rig...don't go into it with expectations of high end pcp performance/accuracy, just have fun doing it.

[Juggernaut12121]

Dang, I guess I'll have to make due with mediocre accuracy
One last (hopefully) question, I can't quite decide between .177 and .22.