Hmmm....
Firstly...pump guns use a spring, and a gas source.
The pump is drawn backwards, in order for a ball to be dropped into teh breech and to cock the hammer. When the pump goes forward, this seat the ball in teh breech with teh bolt.
Apon firing, teh cocked hammer shoots forwards, striking a valve which releases air to power the ball down the barrel.
What I think you are trying to achieve, is a self contained unit, like a break barrel or under leaver or pump up air rifle.
So what you would need, is a spring powered piston, which can be cocked, and then triggered to force a volume of air lareg enough to propel a paintball at around 300fps.
OK. So...were talking about a .68 calibre air rifle.
Air rifles normally use a spring, which when cocked, has a certain volume of air infront of it, which it then compresses on the forward stroke, in order to fire the pellet. Another way to do it, as developed by Theoben (Rolls Royce of Air Rifles), is to use a gas ram, not unlike those found holding up teh hatches on cars. Better than a spring as it doesnt fatigue, and gives a more consitent shove, being a sealed unit.
Problem is...air rifles are usually, .177 or .22 calibre...not .68.
Part of the reason why they are able to fire pellets at speed, is because teh barrel is only very small (.22 or .177), so the air is compressed even further and increases in velocity as it is forced into teh barrel.
So...I suppose you would need an upscaled version of an air rifle.
Ok, so to upscale...we need to know certain things.
Muzzle velocity of air pellets, are normally around 600fps for .22 and around 900 for .177.
Because the .177 is lighter than the .22, it has to travel faster to achive teh same 12ftlb of muzzle energy. Which is why, if you put a .22 barrel on a .177 rifle, you produce over 12ftlb's of energy...not advised.
So...what we need to know is the weight of a .68 calibre paintball.
The formula is something along these lines:
E=m(v2)/450437 (mass times velocity squared, divided by 450437)
E= Muzzle velocity in ftlb's
m= Mass of paintball or pellet in grains
v= Velocity of projectile in feet per second.
We have to back engineer this formula, which im not gonna say I have done loads. Having looked at it...as an example of a .22 pellet at 600 fps, the muzzle energy would be around 11.35 ftlb's.
Ok..so if we times 12 ftlb's by the constant (450437) that gives us 5405244, which must be m(v2) so...if we find v2 which is 300 X 300, that gives us...90000...so by dividing 5405244 by 90000 we should get a maximum weight for a paintball....which is 60 grains or there abouts.
Ok...so...now you know that the .22 pellet was 14.2 grains roughly and your gun needs to proppel a ball of around 60 grains...so your air source needs to be around 4.3 times bigger.
However...my knowledge of physics, does not extend much further than this. I can tell you that size of the area in front of teh piston on an air rifle, is around an inch across and 6 inches long...teh coil spring is around 12 inches long, so must compress to be around 6 inches long itself, behind the piston.
I cant tell you if you would need a piston that is 4 or odd times stronger than an air rifle one, I bloody hope not, as that would be a ******* to cock!
So..erm...Volume of a normal air rifle cylinder would be 25mm diameter and 150mm long, so that pi(r2)h...erm...73.6 cubic centimeters...so...73.6 X 4.3 = 316.8 cubic centimeters.
The bore would have to be around 2.924 inches across (4.3 times .68)..so a bit of 3 inch drainpipe would be a good place to start, and an air rifle mainspring..OX make good ones...you will need a guide for that as well and make a piston and washer for teh front...washer used to be leather, now usually some sort of nylon.
Your spring may need to be quite long, but only as powerful as an air rifle spring, im not sure...would also depend on teh weight of the piston and the friction involved.
Of course it will have to seal quite well, and still overcome friction etc...lube it up...dont allow it to get oil infront of the piston too much or it will diesel, and that could be pretty nasty in this sized gun.
Any help?
Firstly...pump guns use a spring, and a gas source.
The pump is drawn backwards, in order for a ball to be dropped into teh breech and to cock the hammer. When the pump goes forward, this seat the ball in teh breech with teh bolt.
Apon firing, teh cocked hammer shoots forwards, striking a valve which releases air to power the ball down the barrel.
What I think you are trying to achieve, is a self contained unit, like a break barrel or under leaver or pump up air rifle.
So what you would need, is a spring powered piston, which can be cocked, and then triggered to force a volume of air lareg enough to propel a paintball at around 300fps.
OK. So...were talking about a .68 calibre air rifle.
Air rifles normally use a spring, which when cocked, has a certain volume of air infront of it, which it then compresses on the forward stroke, in order to fire the pellet. Another way to do it, as developed by Theoben (Rolls Royce of Air Rifles), is to use a gas ram, not unlike those found holding up teh hatches on cars. Better than a spring as it doesnt fatigue, and gives a more consitent shove, being a sealed unit.
Problem is...air rifles are usually, .177 or .22 calibre...not .68.
Part of the reason why they are able to fire pellets at speed, is because teh barrel is only very small (.22 or .177), so the air is compressed even further and increases in velocity as it is forced into teh barrel.
So...I suppose you would need an upscaled version of an air rifle.
Ok, so to upscale...we need to know certain things.
Muzzle velocity of air pellets, are normally around 600fps for .22 and around 900 for .177.
Because the .177 is lighter than the .22, it has to travel faster to achive teh same 12ftlb of muzzle energy. Which is why, if you put a .22 barrel on a .177 rifle, you produce over 12ftlb's of energy...not advised.
So...what we need to know is the weight of a .68 calibre paintball.
The formula is something along these lines:
E=m(v2)/450437 (mass times velocity squared, divided by 450437)
E= Muzzle velocity in ftlb's
m= Mass of paintball or pellet in grains
v= Velocity of projectile in feet per second.
We have to back engineer this formula, which im not gonna say I have done loads. Having looked at it...as an example of a .22 pellet at 600 fps, the muzzle energy would be around 11.35 ftlb's.
Ok..so if we times 12 ftlb's by the constant (450437) that gives us 5405244, which must be m(v2) so...if we find v2 which is 300 X 300, that gives us...90000...so by dividing 5405244 by 90000 we should get a maximum weight for a paintball....which is 60 grains or there abouts.
Ok...so...now you know that the .22 pellet was 14.2 grains roughly and your gun needs to proppel a ball of around 60 grains...so your air source needs to be around 4.3 times bigger.
However...my knowledge of physics, does not extend much further than this. I can tell you that size of the area in front of teh piston on an air rifle, is around an inch across and 6 inches long...teh coil spring is around 12 inches long, so must compress to be around 6 inches long itself, behind the piston.
I cant tell you if you would need a piston that is 4 or odd times stronger than an air rifle one, I bloody hope not, as that would be a ******* to cock!
So..erm...Volume of a normal air rifle cylinder would be 25mm diameter and 150mm long, so that pi(r2)h...erm...73.6 cubic centimeters...so...73.6 X 4.3 = 316.8 cubic centimeters.
The bore would have to be around 2.924 inches across (4.3 times .68)..so a bit of 3 inch drainpipe would be a good place to start, and an air rifle mainspring..OX make good ones...you will need a guide for that as well and make a piston and washer for teh front...washer used to be leather, now usually some sort of nylon.
Your spring may need to be quite long, but only as powerful as an air rifle spring, im not sure...would also depend on teh weight of the piston and the friction involved.
Of course it will have to seal quite well, and still overcome friction etc...lube it up...dont allow it to get oil infront of the piston too much or it will diesel, and that could be pretty nasty in this sized gun.
Any help?
