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Barrel crown question

A place to ask general spud cannon related questions.
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Barrel crown question

Unread postAuthor: PeteS » Tue Dec 31, 2013 9:13 pm

With target firearms great emphasis is given to barrel crown. Since I am likely to cut spud gun barrels with a hacksaw and by eye, I wonder how much I am impacting accuracy. I do try to clean up the cuts with a belt grinder or file, but I don't typically achieve great precision.

I know that bores, velocity, projectile weight and shape, rifling or lack of rifling, and accuracy expectations are typically much different for spud guns than target rifles. Does that completely or mostly negate concern over finishing barrel cuts on our spud guns?

How much care do you think is necessary or reasonable in this regard? I am guessing that the answers are different depending on the type of launcher in question.
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Re: Barrel crown question

Unread postAuthor: Ragnarok » Wed Jan 01, 2014 5:09 pm

Given the many other issues that exist with spudgun accuracy, I wouldn't expect that a flawless barrel crown is of high importance for most launchers.
If it looks more or less square, it's probably a lot more consistent than the other factors involved.
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Wed Jan 01, 2014 7:01 pm

Ragnarok wrote:If it looks more or less square, it's probably a lot more consistent than the other factors involved.


Agreed. A manual miter saw isn't too expensive though if you want squarer cuts.
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Re: Barrel crown question

Unread postAuthor: jimmy101 » Sat Jan 04, 2014 1:27 pm

Just a wild ass guess but I think there is a chance that the lack of a barrel crown might actually help accuracy a bit in many cases.

If the barrel end is not quite square it'll give the round a bit of a kick in one direction as it exits the barrel. A perfectly square and crowned barrel won't do that. Now if the barrel is un-rifled and the round is aerodynamically unstable (which most shapes are) then it will start to tumble soon after leaving the barrel. In a well-crowned and shaped muzzle the direction of the tumble will be random and change from shot to shot,l which affects accuracy. With a poorly crowned barrel you might just induce a consistent tumble direction in the projectile that would increase accuracy somewhat.
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Sun Jan 05, 2014 6:22 am

jimmy101 wrote:Just a wild ass guess but I think there is a chance that the lack of a barrel crown might actually help accuracy a bit in many cases.


I don't see how a bad crown could help, you're adding another variable...
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Re: Barrel crown question

Unread postAuthor: jimmy101 » Sun Jan 05, 2014 1:53 pm

jackssmirkingrevenge wrote:
jimmy101 wrote:Just a wild ass guess but I think there is a chance that the lack of a barrel crown might actually help accuracy a bit in many cases.


I don't see how a bad crown could help, you're adding another variable...
No, actually you are removing a variable. The bad crown is the same from shot to shot perhaps leading to the same tumble direction from shot to shot. A perfect crown gives no tumble but the round will still tumble, the tumble will just be in a random direction from shot to shot.

A bad crown won't be constant from barrel to barrel but it is a constant for a particular barrel.
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Sun Jan 05, 2014 11:42 pm

jimmy101 wrote:The bad crown is the same from shot to shot perhaps leading to the same tumble direction from shot to shot. A perfect crown gives no tumble but the round will still tumble, the tumble will just be in a random direction from shot to shot.


I see what you're saying but not convinced it would work out in practice. Willing to be proven wrong by experiment though.

Some thing like so:

pneumatic launcher with consistent valve filled to consistent pressure, firing solid cylinders of consistent size and weight, fixed in place.

5 x shots with a 90 degree muzzle angle

5 x shots with a 45 degree muzzle angle

Compare group size.

Any volunteers :D
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Re: Barrel crown question

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Re: Barrel crown question

Unread postAuthor: jimmy101 » Mon Jan 06, 2014 4:28 pm

I'll get right on that. ::rolleyes::

Would you consider a TIPPMAN PB Flatline barrel similar to a crown defect? We all know that rifling improves accuracy. A Flatline barrel indicates that the direction of spin isn't critical, any consitent spin is better than no spin (which is really random spin).

edit: ... or a hop-up
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Mon Jan 06, 2014 8:12 pm

jimmy101 wrote:I'll get right on that.


hehe I was just throwing it open in case anyone had the time or inclination ;)

Would you consider a TIPPMAN PB Flatline barrel similar to a crown defect? We all know that rifling improves accuracy. A Flatline barrel indicates that the direction of spin isn't critical, any consitent spin is better than no spin (which is really random spin).

edit: ... or a hop-up


A bicycle will fall over if still and stay upright when moving, but I'm not sure it is a good analogy.

Hop up, flatline barrels, apex barrels - these all modify the trajectory but I don't think you can say they improve accuracy. They don't make groups tighter - they flatten the trajectory within the projectile's useful range by creating lift though the magnus effect. To my knowledge, gyroscopic stabilisation is not a factor.

Also, this is only relevant to spherical projectiles. Spinning about the axis of travel improves the accuracy of the projectile regardless of whether it is a sphere, cylinder or cone.
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Re: Barrel crown question

Unread postAuthor: Ragnarok » Mon Jan 06, 2014 11:24 pm

jackssmirkingrevenge wrote:To my knowledge, gyroscopic stabilisation is not a factor.

Seriously?

Is it better that the projectile's spin is predictable, and thus avoids generating lift or drag in unanticipated directions? Does the gyroscopic effect achieve such predictability? I'd say yes to both.

The added lift is just another bonus to go with that.
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Tue Jan 07, 2014 3:30 am

Ragnarok wrote:I'd say yes to both.

The added lift is just another bonus to go with that.


I can't seem to find any online tests of group size with or without projectile backspin, care to find any evidence to back that up?

Not a lot of science going in the the airsoft/paintball community, you still get myths like barrel length affecting accuracy because the projectile is guided straight for longer *cringe*

I touched upon it in this thread.

This is one of the best sites I found.

Here's some results with and without hop-up for various BB weights at 500 feet per second.

Image

Looking only at vertical accuracy, you'll see that an unspun projectiles fall in a predictable curve while the ones with hop-up induced backspin drop, then rise, then drop again.

The effect is even more pronounced at 700 feet per second:

Image

If you're trying to hit a man sized target at 150 feet with an airsoft gun, then the trajectory provided by hop-up is just fine.

If you're trying to hit the bullseye on a paper target, then backspin is clearly not helping in this case.
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Re: Barrel crown question

Unread postAuthor: Ragnarok » Tue Jan 07, 2014 11:21 am

jackssmirkingrevenge wrote:I can't seem to find any online tests of group size with or without projectile backspin, care to find any evidence to back that up?

I don't see that it's the kind of thing that needs evidence.
It's self evident that a projectile with consistent spin is going to have a more consistent trajectory than one that's developing a spin at random.

Looking only at vertical accuracy, you'll see that an unspun projectiles fall in a predictable curve while the ones with hop-up induced backspin drop, then rise, then drop again.

That's fallacious though. A more complex trajectory is not inherently unpredictable.

Let's use a decent air rifle as an example of something that's reasonably accurate. But despite the negligible projectile lift, you still have to know the POI offset at different distances in order to get a hit on the bullseye first time.

A trajectory which dips down and up again might be a bit more complex to know the POI for, but if it consistently holds to that trajectory, it could still shoot pretty tight groups - something that's not going to happen if it's spinning off any which way.
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Re: Barrel crown question

Unread postAuthor: 1nxtmonster » Tue Jan 07, 2014 2:41 pm

My experience with spud gun accuracy is that with you standard potato-cutter barrel firing a potato, on high speed camera it goes for about 100 feet then starts to tumble. However, if you are concerned about accuracy enough to consider making a barrel crown, then just improve your projectiles. I used all kinds of front heavy projectiles, like ping pong balls stuffed with clay on a milk lid, hot glue warhead bullets, etc.
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Re: Barrel crown question

Unread postAuthor: Zeus » Tue Jan 07, 2014 5:43 pm

Well I'm an MoA offhand shooter, my vice in the shed aims right out the door to my range, and I can into scientific methods. Name a projectile and I'll build a launcher to suit (read screw on a suitable chamber to a QEV).

I have airsoft BBs in excess, but I doubt they're usable since I'd have to wait for a perfectly still day.
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Re: Barrel crown question

Unread postAuthor: jackssmirkingrevenge » Wed Jan 08, 2014 5:53 am

Ragnarok wrote:I don't see that it's the kind of thing that needs evidence.
It's self evident that a projectile with consistent spin is going to have a more consistent trajectory than one that's developing a spin at random.


Yes, but:

1) how much spin is actually developed at random if it is not artificially induced?

2) how random is it?

A trajectory which dips down and up again might be a bit more complex to know the POI for, but if it consistently holds to that trajectory, it could still shoot pretty tight groups - something that's not going to happen if it's spinning off any which way.


Also true - but I have yet to see the difference in group size between a projectiles fired with the inherent spin from the launcher and with artificially induced spin from a hop-up unit or similar.
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