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Short question topic

A place to ask general spud cannon related questions.
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Re: Short question topic

Unread postAuthor: hectmarr » Thu Jan 18, 2018 9:50 am

What approximate pressure is needed to drive an airsoft ammo of 0.20 gr and a firing barrel of about 15 "at the speed of sound?
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Re: Short question topic

Unread postAuthor: Ragnarok » Thu Jan 18, 2018 12:22 pm

hectmarr wrote:What approximate pressure is needed to drive an airsoft ammo of 0.20 gr and a firing barrel of about 15 "at the speed of sound?

It's not possible to answer that with any degree of accuracy.

When you get near the speed of sound, specifics like gas temperature and valve flow become very significant, to the point that a small error in those values can completely change the result.
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Re: Short question topic

Unread postAuthor: hectmarr » Thu Jan 18, 2018 6:13 pm

Ragnarok wrote:
hectmarr wrote:What approximate pressure is needed to drive an airsoft ammo of 0.20 gr and a firing barrel of about 15 "at the speed of sound?

It's not possible to answer that with any degree of accuracy.

When you get near the speed of sound, specifics like gas temperature and valve flow become very significant, to the point that a small error in those values can completely change the result.

You mean that there is a lack of data in my question, I mean the flow of the valve and the air temperature.
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Re: Short question topic

Unread postAuthor: Cthulhu » Thu Jan 18, 2018 7:44 pm

Really off topic question here, but it's been bothering me for awhile:
I have a daisy 880 pellet gun I'm going to mod, but it has a terrible feeding port that makes putting pellets inside of it difficult. However, the head of the bolt is magnetic...
Is there a good way to add a small magnet at the base of the skirt of each pellet that will be cost effective and not damaging to the rifling in the barrel?
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Re: Short question topic

Unread postAuthor: Ragnarok » Thu Jan 18, 2018 8:29 pm

hectmarr wrote:You mean that there is a lack of data in my question, I mean the flow of the valve and the air temperature.

Yes and no. There's not enough data, but getting good data and doing a broadly accurate calculation with it is impracticable.
The supersonic region is very difficult to predict, as small changes in initial parameters can have a big effect on the final result.

Cthulhu wrote:Is there a good way to add a small magnet at the base of the skirt of each pellet that will be cost effective and not damaging to the rifling in the barrel?

No, because that will significantly negatively impact velocity. You'd basically be building an eddy brake - moving a magnet through a conductive loop (e.g. a metal barrel) creates a current and opposing magnetic field.

I've not actually decided to waste magnets by trying to fire them out of a launcher, but I know that if I drop a stack of neodymium magnets down the same 22mm OD copper tube I use as barrels, they reach an equilibrium (that is, a 1G braking force) at about 16 cm/s. (Taking about 6 seconds to fall through a one metre tube).

This retarding force is directly proportional to the velocity, so at 160 m/s (~520 fps), I'd instead be losing 1000 G of acceleration to eddy braking.
For a back of the envelope calculation, while a launcher like HEAL would fire a non-magnetic projectile of that weight at around 150-160 m/s, I'd be looking at more like 110-120 m/s after the eddy braking was considered, just wasting muzzle energy to generate current of several thousand amps in the barrel.
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Re: Short question topic

Unread postAuthor: Cthulhu » Thu Jan 18, 2018 8:53 pm

I'm not so sure that the magnetic force will do that in this case.
The 880 is advertised as being "dual ammo" (able to shoot bbs and pellets) and the magnetic bolt tip is a feature made to make loading bbs easier (all Daisy steel bbs are magnetic).
I could see it hurting the velocity by adding mass (although that would come with the benefit of increased impact energy), but I don't think it would create an Eddy brake in the steel barrel.
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Re: Short question topic

Unread postAuthor: Ragnarok » Thu Jan 18, 2018 9:06 pm

Cthulhu wrote:The 880 is advertised as being "dual ammo" (able to shoot bbs and pellets) and the magnetic bolt tip is a feature made to make loading bbs easier (all Daisy steel bbs are magnetic).

A magnet in the bolt tip is a completely different case. It's not moving relative to the barrel.

A magnet in the projectile will definitely create an eddy current in the barrel. It's possible the effect won't be severe (it depends on velocity and the strength of the magnet), but it will definitely result in some loss of energy.
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Re: Short question topic

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Re: Short question topic

Unread postAuthor: Cthulhu » Thu Jan 18, 2018 9:26 pm

Ragnarok wrote:
Cthulhu wrote:The 880 is advertised as being "dual ammo" (able to shoot bbs and pellets) and the magnetic bolt tip is a feature made to make loading bbs easier (all Daisy steel bbs are magnetic).

A magnet in the bolt tip is a completely different case. It's not moving relative to the barrel.

A magnet in the projectile will definitely create an eddy current in the barrel. It's possible the effect won't be severe (it depends on velocity and the strength of the magnet), but it will definitely result in some loss of energy.


That's possible. But if bbs go at ~750 fps with their magnetism and pellets go ~715 fps without magnetism, I'm sure it won't be too big of an issue.
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Re: Short question topic

Unread postAuthor: Ragnarok » Thu Jan 18, 2018 9:51 pm

Cthulhu wrote:But if bbs go at ~750 fps with their magnetism...

The BBs are (ferro)magnetic, but they are not a magnet - that's a huge difference in this context.
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Re: Short question topic

Unread postAuthor: Cthulhu » Thu Jan 18, 2018 10:32 pm

Ragnarok wrote:
Cthulhu wrote:But if bbs go at ~750 fps with their magnetism...

The BBs are (ferro)magnetic, but they are not a magnet - that's a huge difference in this context.

Aaaaaaaaaaaaaah, that makes more sense.
Do you think glueing a small piece of mild steel into the base of my pellets will make them easy to load?
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Wed Jan 24, 2018 7:33 am

hectmarr wrote:What approximate pressure is needed to drive an airsoft ammo of 0.20 gr and a firing barrel of about 15 "at the speed of sound?


Dead pictures but I had gone supersonic with this project.

Have you tried GGDT? Not 100% reliable when it comes to supersonic velocities but it can give you a good idea.

Is this for a pneumatic or a hybrid?
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Re: Short question topic

Unread postAuthor: wyz2285 » Thu Jan 25, 2018 12:47 pm

Anyone have read/got an opinion about this?
The fact he/she presumed the pressure in front of the projectile is atmospheric pressure (wouldn't the pressure increase while the projectile accelerates?) and the simulation is done with the projectile been stationary make me doubt the conclusion.
I'm very attempted to use a clear tube to fire a black BB with exaggerated amount of backspin in low speed to see what happens, if I record it at a sharp angle instead perpendicular to the barrel, I might get to see what happens with my Nikon J1 at 1200 fps.
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Re: Short question topic

Unread postAuthor: Anatine Duo » Thu Jan 25, 2018 8:13 pm

Wyz2285 that was a fascinating read. Thank you for posting that.

I don't have the physics background to comment intelligently, but I have been working on ball bearing and marble accuracy and preliminary tests suggest blowby improves accuracy with hard spherical projectiles in hard barrels. I was surprised.

I speculate the blowby is also a significant factor in the pressure ahead of the projectile (I saw high speed video of a burst disk rupturing before the projectile reached it)
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Re: Short question topic

Unread postAuthor: Zeus » Thu Jan 25, 2018 8:43 pm

A lot of factors were ignored, initial projectile position for once, another that this projectile is already spinning as it enters the barrel. They went to a lot of effort, but why go to that much effort when you don't have all the facts.

I personally think, try either, if you're happier with one or the other, then use it!
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Re: Short question topic

Unread postAuthor: jackssmirkingrevenge » Fri Jan 26, 2018 9:02 am

My general opinion is that while a hop-up system makes airsoft playable in terms of handguns and assault rifles, there are too many variables in the way the spin is imparted for it to lead to meaningful accuracy at long range. Apart from that, inducing the spin robs the BB of energy that reduces its impact even if it does manage to connect with the target. The ultimate sniper system would combine accurate range estimation along with appropriate drop compensation.

The Airsoft Trajectory Project is also an interesting link.
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