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“Liquid” gas powered longrifle

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Lentamentalisk » Tue Apr 29, 2008 9:49 am

I'm not sure if you guys noticed it, but i think its in BTTF3 that doc shows up in the car and starts digging through the recycling for soda cans. It recently occurred to me that he changed the power source to a fuel cell, and was mixing the aluminum with lye to create hydrogen! Little did you know, BTTF actually directly relates to spuddin'!
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Unread postAuthor: judgment_arms » Tue Apr 29, 2008 9:55 am

Wrong, that’s the end of part 1, and it’s a “Mr. Fusion”, uses common household waste to generate electricity (the 1.21 jigawats to run the time circuits), if you watch he also throws in some other junk including an egg carton. :wink:

I’ve seen the moves far to many times… I think later I’ll pop them into the VCR and watch them again… :roll: :D

Nuts, the old record of the BTTF sound track is skipping…
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Unread postAuthor: Ragnarok » Tue Apr 29, 2008 2:03 pm

judgment_arms wrote:(if you want the exact number I’ll pop Back to the Future I in the VCR and find out)

No need. I have a clip on my hard drive from when I was experimenting with creating the time travel effect in photoshop.
The counter on the radio control actually stops counting up at 88.0 - goes rapidly until then, then stops at exactly 88.
Presumably because that's as far as the real-life counting circuit that controlled that was made to go.

Being pedantic, a burnout is just about the worst possible way to get up to speed as fast as possible - although it does look cool. Hollywood science I guess.
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Unread postAuthor: Fnord » Tue Apr 29, 2008 2:09 pm

Hollywood science I guess.


Just a word of warning, the last time you examined hollywood science in an on-topic thread we ended up hunting raptors.
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Unread postAuthor: Ragnarok » Tue Apr 29, 2008 2:17 pm

_Fnord wrote:Just a word of warning, the last time you examined hollywood science in an on-topic thread we ended up hunting raptors.

I would suspect that no-one in the whole of history has ever said that before - This can't make any sense to people who haven't been here that long.
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Unread postAuthor: judgment_arms » Tue Apr 29, 2008 3:35 pm

I did a search on “hunting raptors” only two threads came up… wonder why?

I’d just like to add my two cents to the now dead discussion:
Best gun for raptors: M79 and a ton of frag and HE rounds. :D
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Since the original topic has long sense died, I’m perfectly fine with a new topic being discussed in this thread, particularly one as entertaining as Hollywood science, or lack there of.

Anyways, the readout on the controller only goes up to 88, doesn’t show fractions at all.
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Unread postAuthor: bluerussetboy » Thu May 01, 2008 5:15 pm

judgment_arms wrote:Bluerussetboy, by “jumping” I mean being hammered so hard be the air that the wadded paintball skips right over the shallow groves of the rifling, with a 1 in 14 rate of twist it’s a vary real possibility.


judgment_arms wrote:I have already made a prototype paintball rifle that had a bore rifled to 1-in-102 and about 18.5 inches long, according to the GGDT it ws shooting at 302fps and I saw a slight long range improvement in accuracy over my stock Tippann 98c.
Ragnarok brought it to my attention that 1-in-102 was far to slow if a twist rate to stabilize the paintball properly so my new barrel will have the much more appropriate ROT of 1-in-14.


How exactly have you and Ragnarok come up with this more appropriate rate of twist? I know that the Greenhill formula doesn't really work with round ball.

For example, most muzzleloaders today come with a 1:48 twist, but for round ball accuracy you require a 1:66 twist. Conical bullets prefer a faster twist of 1:28. These are in barrels of 26"-28". These rifling rates are acceptable in .50, .54 and .58 calibers.
I do have a .75 Brown Bess, it is a smoothbore flintlock. It is quite accurate to 75 yards.

1:14 is too fast of a twist rate for round projectiles. 1:66 is probably a better choice.
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Unread postAuthor: Ragnarok » Thu May 01, 2008 6:01 pm

bluerussetboy wrote:How exactly have you and Ragnarok come up with this more appropriate rate of twist? I know that the Greenhill formula doesn't really work with round ball.

Indeed, you are right, the Greenhill formula is not ideally suited for round ball. In fact, it's an outdated convention for any form of projectile, and no modern ballistics expert worth his/her salt would use it.

We however, are not blessed with the vast budgets of the ammo companies, and we don't have the sophisticated technology.
Given our limited resources, the Greenhill formula is perfectly sufficient.

For example, most muzzleloaders today come with a 1:48 twist, but for round ball accuracy you require a 1:66 twist.
Conical bullets prefer a faster twist of 1:28.

You can't generalise like that. Many conical bullets "like" a rate of twist measured at maybe 7" or 8" - some will use very long rifling.
Show a 1:28 twist to the notorious SS109 bullet, and you'll get groups the size of pie plates at spitting ranges.

1:14 is too fast of a twist rate for round projectiles. 1:66 is probably a better choice.

Slow projectiles need steeper rifling, as do less dense ones.
Compared to your typical lead ball, a paintball is very slow, and exceptionally low density, therefore requires steeper twist.

You can't apply the ideal rifling rate of a specific lead ball (and there is only one size of lead ball for which 1:66 is the ideal rate), and apply it to a very slow, very light paintball.
That's roughly the same as saying the ideal engine size for a city run-about car is around 1 litre, so you should put 1 litre engines in all muscle cars too.
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Unread postAuthor: bluerussetboy » Thu May 01, 2008 7:10 pm

I was using muzzleloaders as an example and not 5.56x45mm NATO round. A .68 cal paintball or even a .50 cal paintball has more in common with a muzzleloader than a 5.56x45mm NATO round. I made no generalizations.

Also typically your slower projectiles have a slower(not steeper or faster) twist rate. A simple comparison of muzzleloader velocities to your 5.56x45mm velocity and a comparison of twist rates will show that.

Diameter is the largest constant key factor in rifle twist rates. This can easily be found by comparing the various calibers around the world with their twist rates.

As for the low density, that is pretty much the only part of your argument that I can't dispute with hard data.

Ragnarok wrote:You can't apply the ideal rifling rate of a specific lead ball (and there is only one size of lead ball for which 1:66 is the ideal rate), and apply it to a very slow, very light paintball.
That's roughly the same as saying the ideal engine size for a city run-about car is around 1 litre, so you should put 1 litre engines in all muscle cars too.


Actually .50 cal and .54 cal show close enough accuracy in a 1:66 twist to call it ideal, .58 tends to fly a little. I actually have several chamberings with the 1:66 twist.

I only compare dump trucks to dump trucks. Doesn't matter if it's a Hino, a Mac, or a Peterbuilt. :wink:
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Unread postAuthor: Ragnarok » Thu May 01, 2008 7:31 pm

bluerussetboy wrote:Typically your slower projectiles have a slower(not steeper or faster) twist rate. A simple comparison of muzzleloader velocities to your 5.56x45mm velocity and a comparison of twist rates will show that.

Then why would the greenhill formula have a term that lengthens the twist the fast the projectile goes?

3.5 * sqrt(Velocity) * Diameter<sup>2</sup>/ Length * SQRT(Density/10.9) = inches per turn.

As you can see, the faster the projectile, the longer the rifling is.

As for the low density, that is pretty much the only part of your argument that I can't dispute with hard data.

And it's an important one. Where lead has a density of 11.3 g/cubic cm, a paintball is only around a ninth of that density.
All else aside, feeding that into the greenhill formula makes the rifling three times tighter (as it is related to the square root of the density).

Actually .50 cal and .54 cal show close enough accuracy in a 1:66 twist to call it ideal, .58 tends to fly a little. I actually have several chamberings with the 1:66 twist.

Ideal rifling is a bit of a tricky one. Theoretically, ideal is the slowest possible rate at which an acceptable stability is reached.
Steeper than that will slow the projectile, add a whole load of other problems, and more than that, when the projectile slows from drag, the slower spin will help maintain stability.

So you can't really use the word ideal, but I see the point.

I of course, am not an expert on your average muzzleloader, having only ever fired .22 LR, Nato 5.56mm and 12 gauge - and that's three more varieties than most Brits.
For those reasons, I shall take your word on the performance of the rifling in the muzzle loaders. That doesn't however mean that the results have to translate to paintballs.

I only compare dump trucks to dump trucks. Doesn't matter if it's a Hino, a Mac, or a Peterbuilt. :wink:

But if the thing in question is a kid's toy dump truck...

High velocity, high density is a completely different world to low velocity, low density.
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Unread postAuthor: bluerussetboy » Thu May 01, 2008 8:51 pm

Ragnarok wrote:
bluerussetboy wrote:Typically your slower projectiles have a slower(not steeper or faster) twist rate. A simple comparison of muzzleloader velocities to your 5.56x45mm velocity and a comparison of twist rates will show that.

Then why would the greenhill formula have a term that lengthens the twist the fast the projectile goes?

3.5 * sqrt(Velocity) * Diameter<sup>2</sup>/ Length * SQRT(Density/10.9) = inches per turn.

As you can see, the faster the projectile, the longer the rifling is.

My fault for not making a better distinction in my comparison. Typically your larger slower calibers(ie. .54 cal @ 1680 fps) have a slower twist rate than your smaller faster calibers(ie. 5.56x45mm @ 3100 fps).

And yes for a given caliber if you increase the velocity the twist rate lengthens(slows).

While paintball guns and muzzleloaders are not identical, they are close enough in function to where we can use some of the same principals. Given the vast amount of R&D done by major firearm and ammunition companies we should still be able to make a halfway educated guess. Research has shown that in any given caliber a twist rate that is to steep/fast has the ability to exaggerate a projectile's concentricity(sp?) resulting in 'wobble'. Jacketed bullets have been known to shed their copper or even disintegrate due to the greater centrifugal force of a too fast twist rate. Given the nature of a paintball, I would opt for a slower twist rate.
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Unread postAuthor: judgment_arms » Thu May 01, 2008 9:01 pm

Well, since I’ll have a 1-in-60 blank, I suppose that I could make a test barrel with that and see how it compares.
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Unread postAuthor: mobile chernobyl » Thu May 01, 2008 11:30 pm

judgment_arms wrote:How come every time I bring up my paintball rifle project there’s always someone who has to make the argument that you can’t rifle a paintball…


Sorry for my somewhat offensive response it was late and i was deranged from being up for so long, so alot of my "facts" are just copy paste crap from paintball forums.

My argument is not than you cant induce a spin on a paintball either by rifiling ,bent barrels or rubber wedges. Sure you can, its easier than you think, and its often too easy. What I'm trying to say is simply that among all the other inconsistentcies you will encounter with paintballs, rifling should be the least of your worries for accuracy, bore match should be higher up, and then you wont need to fire it as a PRB projectile, allowing you to shorten the time between shots, but that does not even seem to be a concern with you and you're going for authenticity I geuss. I hear theres a good deal on exotic siberian possum fur sniper hats right now. :wink:
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Unread postAuthor: THUNDERLORD » Thu May 01, 2008 11:43 pm

judgment_arms wrote:Well, since I’ll have a 1-in-60 blank, I suppose that I could make a test barrel with that and see how it compares.


That's cool since you plan on selling rifled barrels. :roll:
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