Posted: Mon Jun 08, 2009 2:24 pm
To get a projectile out to that kind of range with a moderately flat trajectory, power is essential. A good projectile design will allow you to get away with less of it however.john bunsenburner wrote:Would you recommend working more on the projectile design or the gun design, which one of the two is more important, easier?
For the 400 metre ranges I'm looking at, I'm using velocities of around 270 m/s (~900 fps), and a dart with a ballistic coefficient of ~5000 kg/m<sup>2</sup>. For comparison, a really good .308" bullet might be 2000 kg/m<sup>2</sup> (Bear in mind, unlike drag coefficient, higher is better with ballistic coefficient).
To put that into a slightly more useful figure, the dart's BC means it'll travel the 400 metres with ~95% velocity retention.
In short, those ranges take very respectable velocity, towards the practical limits of what pneumatics can manage, even with BCs as good as we're discussing. Even in this case, I think my set up is reasonably marginal as far as actually getting the projectile out there with a flat trajectory. It'll still take a ~1.6 degree launch angle, which is pretty big as far as this kind of thing is concerned - outside of the adjustment range of most scopes, so it'll need some form of shimming.
Now, if you want to use commercial pellets, there's a problem there. Air rifle pellets are about 100 kg/m<sup>2</sup> in domed .177" and 150 kg/m<sup>2</sup> in domed .22", which seriously puts restrictions on the potential range.
A good commercial air rifle might put either calibre to 900-1000 fps while still keeping decent accuracy, but even so, reasonably accurate ranges are limited to within 200 metres. And that of course assumes you can make those velocities in the first place.
Really, you want to be making at least 800 fps, and you'll need a pretty impressive ballistic coefficient to go with that. That is to say, both things are important.
Bear in mind though, high ballistic coefficient projectiles can still take quite a lot of power to accelerate, because they've got to have some mass to make up a respectable sectional density.