Rifling rates for potatoes?

[boilingleadbath]

I apologize if this would be better suited for "general discussion", but I felt that scence it had to do with "percision" shooting I'd put it here.
Feel free to move it.
Anyway, adapting the contents from my posts in another forum that told me to seek out somewhere more technical...

I was considering making a low-quality rifling machine for cutting grooves in PVC, for the use in spudguns.

I'm a bit stumped on the rifling rate to use, though; the greenhill formula certainly isn't going to give me accurate results (being intended for pointed lead projectiles 6-10 times faster).

In addition, I've been reading about the effects of rifling a bit, and it leaves me confused. Is an overstabilized bullet really that much less accurate?

Later, I drew a couple conclusions, if you want to tell me how wrong they are...

1) With a group size of 35 MOA even with a riffled barrel, I'm not sure that the over-stabilization wobble is an important effect.

2) Thus, it won't hurt (ignoring tractablitiy) to over-stabilize the projectile.

3) Thus, I'd want to go faster than the riffling rate of 1:400" that would be sufficient for a 159 caliper 2" long projectile as predicted bythis spreadsheet

4) So, maybe a 1:120" or so for a stability factor of 11? (Acctualy higher, as the overturning force would be lower, right?)

I don't need answears with 100% certainty, and certainly don't expect them. Educated speculation is quite fine.

Here's some data on the projectiles:
Diameter: 1.59"/2.06" (1.5" pipe, 2" pipe)
length: aprox. 2"
density: aprox. 1.1 g/cm^3
velocity: aprox. 300-500 fps, generaly
general shape: cylinder capped with rounded ends as in this. (of course, potatoes arn't spheres, so that's not a perfect impersonation, but it gives you an idea)

If it helps, 1.5" barrels rifled with a 1:60 twist shrinks the mean radius at 50 yards from about 26" (13 shot) to about 13.5" (11 shot).
That's using these barrels and this testing procudure.
No, that's NOT a typo. These things really are that inaccurate... I'm thinking that stiffening the barrel would significantly help get the group sizes down too. High-speed movies show the muzzle vibrating by maybe an inch!
I'm planning on checking the effect of stiffening it eventualy, too.

 

[IMtheNRA]

Maybe this site can help. I see they have a forum there, but I did not check it out. Good luck and let us know what potato you'd recommend for bears. My money is on Yukon Gold.

http://www.spudtech.com/default.asp

 

[AJAX22]

I don't know about rifling rates, I suspect that as the velocity goes down, the rate of twist should go up.

focus on standardizing your projectile first, make a cookie cutter type setup to cut cylinders out of potatoes and make a large pencil sharpener out of a wood block and a kitchen knife to put a uniform cone on the end of the cylinder.

then decide on a standard weight and try to get the potatos to be roughly the same.

the really tricky part will be to get the propellant the same every time. You might have good luck using a pneumatic setup.

also I've heard that freezing the potato can help.

I haven't done this myself, my potato launching experiances are limited to healping some guys in my dorm build a setup (they later were arrested with it, but thats a different story)

for the projectile cutter we used a 1.5 inch steel pipe that was sharpened on a grinder, and a 4x4 block to make the giant pencil sharpener, we used about a 30 degree angle for the blade, and shaped the block with a jigsaw and a belt sander. In hindsight the block should have been a bit bigger. we used an old carving knife from the swapmeet for the blade.

We had some strange looks while working on it in the aerospace labs on campus, but no one said anything.

 

[AJAX22]

Here's the link directly to the rifled barrels from the link above.


http://www.spudtech.com/detail.asp?id=32

save you alot of time just buyin one, probably cheaper too.

 

[gezzer]

A potato plug will be just better than a round ball. Same as any muzzleloader 1 in 70 for round ball, 1 in 48 for rb and short bullets, if you are making a magnum using 6 inch long Idaho’s go with the 1in 24 twist.

I would not go to deep with the groove and try and make your cutter with rounded corners in the bottom of the groove to keep stress points down so you will not have sharp plastic shards penetrating your flesh

Sounds like a fun project!!

 

[boilingleadbath]

Replies

Funny that you mention the spudtech forums.
That was the first place I asked - discussion basicaly went off topic into "OMG, multimillion dollar 5-axis mills are so cool!". In other words, they didn't have a clue.
The second was a-place-that-shalt-remain-unnamed, where they indeed pointed out that one can buy rifled barrels... and at least they recognised that they didn't really know.

And I'm aware that one can buy the barrels online.
Turns out, though, that I can screw up my twist guide twice and still come out ahead... although I'm more doing this to see if I can than anything else.

Addressing replies on this forum, however...

AJAX22, the bit about rifling rates having to increase as muzzle velocity goes down makes sence, and I've read things that essentialy say that it does. (though typicaly phrased the other way around and relating to the 150/180 constant in greenhill's formula)

Onto standardization, the idea of building a "pencil sharpener" is really neat. I guess that's another thing I'll have to experiment with.
Acctualy, I'd say that velocity standardation in (combustion) spudguns is the easy part. Assuming that one has a fuel-air mixing device and a device to inject the same amount of fuel each time, velocity variations can be brought below 1% (std. dev.) using consistent ammunition. I forget the numbers relating to consistency in pneumatics, but they arn't much - if any - better.

Gezzer, looking at greenhill's formula (150 * D^2/L), it it appears that larger diameter projectiles with the same aspect ratio (L/D) are better. Is this wrong?

I geuss, thinking about it more, that my real question is "how much will overstabilizing the projectile really hurt my group sizes, considering my ammunition"?
That's assuming that the spreadsheet from 22-250.com is accurate at such low speeds. It might be, but the formulas it's using seem quite a bit less complicated than what I've seen elsewhere on the web.

 

[Vern Humphrey]

Use the straight Greenhill formula -- it will be close enough.

(Bullet diameter^2 X 150)/bullet length.

Overstabilization will not be a problem (it never is.) The problem will be mechanical failure of the projectile (shredding on the rifling) which will occur long before you even begin to experience trail.

 

[boilingleadbath]

What's "trail"?

 

[ArmandTanzarian]

I was considering making a low-quality rifling machine for cutting grooves in PVC, for the use in spudguns.

This thread just makes my day. Only on THR would peeps be talkin about rifling their spudguns for accuracy!

 

[duck911]

Try a greased patch to keep the spud from shredding on the rifling.

Ok, it may not help much, but at least you're halfway to some decent hash browns!

 

[Vern Humphrey]

What's "trail"?

Trail is when the axis of the bullet is not parallel to the direction of flight. It usually occurs at high angles of fire, where the trajectory makes an abrupt change in direction, and the gyroscopically-stabilized bullet remains pointing in the original direction.

Trail is the first sign of "overstabilization."