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Hi everybody, i'm new in the forum and i need your help to solve a problem that is driving me crazy: i bought a qev of this tipe https://www.google.it/search?hl=it&q=qe ... tmyPZoM%3A
And i put it on my gun. I have got a chamber with a valve that takes a part of the air from it. The air has to pass through a tiny hole that slows the flow, because the air goes into the qev from the barrel side and not from behind the diaphram. I cut the diaphram in some points to let the air pass with a small flow. Of course there is a spring behind the diaphram, on the pilot side. The problem is that the diaphram closes the barrel hole only for the first shot, then when i open the chamber valve to shoot again, the air passes easily through the barrel. It happened only once that i could shoot all the air reserve without any problems. I guess it is a problem of the spring but i can't solve it. I use a pressure of 12-13 bars. What do you think? Thank you in advance
I think I understand your question but it would be better to see your set up, puoi farci una foto?
these are the photos
I think the problem is that the grooves in the piston are insufficient for air to flow past it.
Have you tried poking a small pin-hole instead? This is the normal solution when people are using chamber-filling QEVs.
It's hard to say exactly from those pictures, but I have a few thoughts. I don't know if its the quality of the pic but your sealing face looks very rough, almost like you hit it with sandpaper. Next, in my opinion, you have too many equalization holes, and also they're in the wrong place. I have seen this setup work well with one tiny hole in the thick portion of the piston, off to the side of the sealing face. Your hole(s) are actually on the sliding edge of the piston which probably affects the smoothness of it's travel. Hope this helps.
Ok, thank you very much. I will buy a new piston and drill the holes in the thick part. Then i'll let you know..
Can i ask you some other things? The volume of the chamber for each shot is more or less 20-21 ml. What do you think is the right lenght for the barrel if i the diameter is 5.5 mm and i want to shoot diabolo pellets? And do you think that i can go more than 15 bars with the pressure with that kind of valve? The seller told me not to exceed 10 bars.. In your opinion could a pellet of one gram go supersonic with that pression?
Hey, just in case we arent on the same page as to where to drill ur tiny hole:
This is someone who has a similar launcher.
The setup should go pretty much the same as yours.
As far as pellets I have no idea, but your QEV valve should easily be good for 400 psi, or 27.5 bars. The chamber pressure is another story, but I would trust the valve to 400.
Edit: Fast forward to 3:50 ON THE VID.
Edit, not likely at 15 bars to get supersonic. maybe 700 fps is reachable though.
Last edited by jrrdw on Sun Oct 20, 2013 12:56 pm, edited 1 time in total.
Reason: Fixed video.
This is a fairly good volume for the small calibre.
People have taken this sort of valve to 30 bar without any problems.
With this project using 25 bar and a burst disk (disco di rottura) valve I got just over 700 feet per second with a 5.5mm pellet, with your setup my rough estimate with a 50cm barrel would be around 400-500 feet per second.
If you want semi automatic, instead of poking holes in the burst disk, have you considered a three way valve as a pilot? It's a more efficient and neater setup.
Thank you jack. I've thought about semiauto, but I'm trying to build a sniper rifle, so i'd privilege power and precision over the other aspects. The problem of the semiauto is that I cant build a recharge mechanism for pellets without a big air loss, unless you help me.. Regarding the valve right now it is a kind of semi auto, because it takes about three seconds for the air to refill the valve, so actually i don't need any valve in the chamber and i can shoot every three-four seconds. An other issue is the barrel. I bought a smooth aluminium barrel, but i'd like to rifle it to give the spin to the pellet. To you think that's possible?
For power from small calibres, pressure is vital. Remember that commercially available pneumatic rifles operate in the 150-250 bar range. I would definitely think about using either a bicycle shock absorber pump or fridge compressor to get higher pressures in the 25-30 bar range.
This method is simple and with minimal air loss or dead volume, make a hole in the barrel covered with a sleeve, something likethis golf ball launcher:
It's not good for fast repeating shots but it allows for efficient breech loading.
Making your own rifling is possible but a bit of a challenge, the easiest is to buy one ready made. Look for a supplier of a cheap airgun brand (like Crosman or Gamo for example) and see if they have replacement barrels.
This is a 0.177" barrel you can get from a dealer in the UK for example: https://www.gunspares.co.uk/shopdisplay ... 0&cat=1077
For only 25 euro, its a much better idea to buy one that to waste time and effort making your own.
Note however that medium weight diablo pellets are generally stable in flight without the need for rifling, so you should still get decent accuracy as long as the barrel is a tight fit (for example using a 6mm barrel with 5.5mm pellets will not be accurate).
I made the same mechanism to recharge the ammo in my old gun, but it doesn't make sense to have a semi auto valve and then need to open the sleeve every time. i need an efficient way to recharge pellets quickly without air loss.
PS I don't think i'm going to rifle the barrel..too challenging
Something like this perhaps.
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