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Long range accuracy

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Long range accuracy

Unread postAuthor: jstntmcdrmtt » Tue Jan 14, 2014 6:52 pm

Hello, folks. I'm new to the forum, and am currently in the "seriously considering" phase of building my first hybrid. Now, I've spent the last few days snooping around in the shadows-looking for all my logistical answers; "how do you hold a burst disk?", "what's a good chamber to barrel ratio?", "what's all this I hear about manometric fueling?", ".....ignition?" and so on and so forth. So I'll try not to bother you all with that stuff until I encounter problems.

Throughout my time researching the forums I've been impressed with the thoroughness in the designs of some of the launchers, impressed at the creative ingenuity of some of the others, and thrown into giggle fits by watching a multitude of different commonly found or purpose-engineered projectiles bash through wooden furniture, pierce household appliances, and penetrate thick sheets of steel as if they were warm butter..but all the while I found something missing..

anyway. about a fortnight ago, late at night while I was sleeping, a vision came to my head. I was standing on some steps, about three hundred and twenty three yards from a life-sized bear statue...I shouldered a hybrid potato cannon, peered down the sights, and then let fly a heavy slug which totally NAILED that bugger! :drunken:

does anyone have experience trying to make these launchers hit distant targets reliably? I'm hoping someone has done some experimenting with this breed of precision marksmanship and can reassure me that it's possible..and maybe share some techniques they used which made it possible..barrels, ignition, projectiles and so forth. It may be that my dream will be an enormous challenge to realize-or maybe I'm talking about a run-of the mill hybrid with a some improvised iron sights. Thoughts?

Designs to follow--hopefully including some suggestions from more experienced fellows
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Re: Long range accuracy

Unread postAuthor: jackssmirkingrevenge » Tue Jan 14, 2014 8:51 pm

jstntmcdrmtt wrote:Hello, folks. I'm new to the forum, and am currently in the "seriously considering" phase of building my first hybrid.


Jumping in the deep end eh :)

does anyone have experience trying to make these launchers hit distant targets reliably? I'm hoping someone has done some experimenting with this breed of precision marksmanship and can reassure me that it's possible..and maybe share some techniques they used which made it possible..barrels, ignition, projectiles and so forth. It may be that my dream will be an enormous challenge to realize-or maybe I'm talking about a run-of the mill hybrid with a some improvised iron sights. Thoughts?


One word, consistency. You want projectiles that are the same shape, size and weight, and a launcher that launches them at the same velocity every time.

For a hybrid, this would mean a uniform burst disk material held in a repeatable clamp, an accurate filling system that can fill to the same pressure and fuel concentration every time and a consistent ignition system.

In terms of projectiles, if you're serious about accuracy you might want to get some professionally made ones if you don't have your own machining facilities...

A low drag projectile with high density will fly flatter and make it easier to hit target at distance.

In a nutshell, hitting a a life-sized bear statue from the oddly specific distance of three hundred and twenty three yards is possible - but it is going to be a challenge and likely not one that can be completed on a low budget.
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Re: Long range accuracy

Unread postAuthor: Ragnarok » Wed Jan 15, 2014 5:34 pm

I've probably thought about this a bit more than most, given I put some time a few years ago into a similar objective - hitting (and piercing) steel plates at 400 metres.
Unfortunately, it had to be abandoned when it became clear I wouldn't have anywhere safe to test that full on objective, so it never got much beyond groundwork and smaller scale testing. Still, there was a degree of promise.

The big keyword, as Jack says, is consistency. That's may not apply to just projectile shape, size and mass, or muzzle velocity. Factors like barrel harmonics and muzzle blast may also be relevant.

To waffle on a couple of other factors...

~~~~~

The choice of velocity:

It's not just important that velocity is consistent - it also needs to be high. To get something out to 300 metres or so with a reasonably flat trajectory, your projectile needs to be clocking a fair rate of knots.

Bare minimum muzzle velocity (neglecting drag entirely) for a 1 degree firing angle to get to the 300 metres (you said 295, but I'm rounding off) would be 290 m/s. You'll need at least a bit more to compensate for drag losses, and more again the more inefficient the projectile.

The problem with that kind of velocity is that it's entering the transonic range for most projectile shapes. Transonic velocities, when the airflow over the projectile is supersonic in some places and subsonic in others, result in a very large and fairly sharp peak in the projectile's drag coefficient.

This is bad, not just because it wastes loads of kinetic energy, but because it makes the projectile hard to predict.
Small changes in temperature can change the local speed of sound and ambient wind can change the airspeed velocity over the projectile; factors that can be significant in the transonic region, as small changes in the Mach number of the air over the projectile can change the drag coefficient by a large factor.
This can change its velocity dramatically over the whole trajectory, and thus where that trajectory goes.

So, if you're firing sub-sonic, you probably want to be firing a little slower than that. Much more than about Mach 0.8 (which is normally about ~270 m/s) isn't a great idea.
Of course, this means a somewhat more "lobbed" shot, which makes range estimation very important - and probably needs a dose of computer simulation to aid calibration for that day's ambient weather.

The alternative is of course a supersonic projectile - but just like it's best to be sub-sonic by a reasonable degree, you really want to be supersonic by a fair margin.
Partly because the transonic region continues up to about Mach 1.2 for many projectiles, partly because the projectile's own sonic boom catching up with it doesn't do good things for projectile stability.

A hybrid can be powerful enough to do this, but achieving a muzzle velocity of significantly past 400 m/s with a moderately heavy projectile will need a fair deal of grunt.
That said, if you can do it, it will make exact range estimation much less important.

~~~~~

Projectile choice:

You're just NOT getting to those distances accurately without a stable projectile, and rifling is an art that's never really been cracked for spudguns.

This means the only real choice is drag stabilisation, in the form of either a dart (like Depleted Uranium anti-tank projectiles) or a nose heavy slug (like Foster or Brenneke produce for shotguns). For a given mass, the former will be considerably more aerodynamic and penetrate better; the latter will probably produce visibly more destructive effects on target.

Although you've mentioned interest in slugs, I'd definitely recommend the former, as its better aerodynamic efficiency will make it far easier to actually get to the target.
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Re: Long range accuracy

Unread postAuthor: Gippeto » Wed Jan 15, 2014 10:03 pm

Hybrid? Meh. :roll:

Theory and conjecture? Meh.

High pressure pneumatics with rifled barrels shooting cast lead slugs...judge for yourself. 8)

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Re: Long range accuracy

Unread postAuthor: jackssmirkingrevenge » Thu Jan 16, 2014 7:51 pm

Gippeto wrote:High pressure pneumatics with rifled barrels shooting cast lead slugs...judge for yourself. 8)


Impressive!

It proves the point though, rifling for stability, carefully made projectiles and a consistent valve and pressure are what make this possible.
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Re: Long range accuracy

Unread postAuthor: Gippeto » Fri Jan 17, 2014 12:50 am

It proves that "conventional" works.

I admit to having doubts as to whether a hybrid can reliably produce consistent velocity (ie....+/- 5% or less).

Perhaps a piston hybrid....but I doubt it. Someone feel free to prove me wrong. :)
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