Coax steel bb gun

[TheCisco_kid]

This is the second coaxial gun I've made, and this one is all metal except for a couple of jb epoxy welds.

The barrel is 1/4 steel brake line bought from an auto parts store, which is the perfect diameter for BB's. The body and chamber is 1/2 inch steel pipe. I used two rubber stoppers glued together for the piston which fits perfectly in the pipe. Originally I exhausted and filled from a 1/4 inch ball valve, but I had this qev around not doing anything so I added that as my exhaust valve. For ammo I'm using bbs and some homemade darts out of qtips and the ends of nails tapped together. I strapped the gun yo an old crutch for a stock and it works quite well Here's some pics:

It's fairly accurate. At 40 feet here's the grouping:

[jackssmirkingrevenge]

It's fairly accurate. At 40 feet here's the grouping

Definitely impressive given the context!

1/4" steel bearings would probably give even better accuracy and should be relatively cheap to obtain.

[Cthulhu]

I've never made a coaxial, does a faster release such as the qev over the ball valve really improve performance?

[jackssmirkingrevenge]

I've never made a coaxial, does a faster release such as the qev over the ball valve really improve performance?

To be honest in this case you could have used the QEV as a direct valve triggered by a blowgun and you would have got the same performance.

If you want more power, more pressure is key, even for the same quantity of air. Something I always link to that illustrates the point, here is the data for a virtual launcher modeled in GGDT with a progressively smaller chamber filled to a higher pressure - in each case, we're talking about the same quantity of air, but in a reduced volume and therefore at higher pressure.

20 inch chamber at 50 psi - 406 feet per second - 2 ft lbs

10 inch chamber at 100 psi - 537 feet per second - 3 ft lbs

5 inch chamber at 200 psi - 684 feet per second - 6 ft lbs

2.5 inch chamber at 400 psi - 830 feet per second - 8 ft lbs

1.25 inch chamber at 800 psi - 958 feet per second - 11 ft lbs

0.625 inch chamber at 1600 psi - 1006 feet per second - 12 ft lbs

[TheCisco_kid]

It's fairly accurate. At 40 feet here's the grouping

Definitely impressive given the context!

1/4" steel bearings would probably give even better accuracy and should be relatively cheap to obtain.

Well I think it's quarter inch outer diameter, which, in this case gives the barrel an inner diamter thats just a bit larger than .177 cal. 5/8s brake line fits those quarter inch ball bearings a lot better if I remember correctly, but I may be completely wrong.


And yeah, I originally exhausted with only the quarter inch ball valve, but I just slapped the qev on for giggles and like has been said there's not any gained performance. I just like it

If I want higher pressure though, I do have a shock pump and use that as a boost after filling it to 100 psi with my compressor. So far I've only gone up to around 250 psi and noticed a huge boost in power from the shots without the shock pump. Pressure really is the key like JSR has said.

[jackssmirkingrevenge]

Well I think it's quarter inch outer diameter, which, in this case gives the barrel an inner diamter thats just a bit larger than .177 cal.

Ah, I thought you were referring to the ID.