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need help determining twist rate

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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need help determining twist rate

Unread postAuthor: far_cry » Mon Jul 16, 2012 11:08 am

i want to try rifling an 8mm tube .
i know that rifling twist rate is not easy to calculate ,so here some spec and info that can help.

barrel ID 8mm
barrel length 60cm
ammo ,i think for plastic ammo that wight about 1 gram ~
pressure used 40-100 bar

so what twist rate should i done
how much groves ,4,6 ?

as i know air rifle barrel dont have deep grove ,is that correct ?

if you need another info just ask
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Unread postAuthor: Gippeto » Mon Jul 16, 2012 11:24 am

http://kwk.us/twist.html

Typical groove depth is ~ .002"-.003".

LW barrels have 12 grooves...Crosman barrels use 10.

Want some pics if you do this. :)
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Unread postAuthor: velocity3x » Mon Jul 16, 2012 11:33 am

A typical twist rate for 30 cal rifles is 1:10, but their projectiles are much heavier.
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 16, 2012 12:36 pm

edit: nevermind, Gippeto beat me to it :roll:
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Unread postAuthor: frontstuffer » Mon Jul 16, 2012 1:20 pm

Twist rate depends on velocity and length and caliber of projectile. Without knowing these there is no way to calculate twist. Most small caliber air rifles are around 1 in 16" rifling at around 700 fps. Longer projectiles need a faster twist than shorter. Twist rate is not weight dependent. Also round projectiles such as lead round balls use a slower twist rate than elongated projectiles, but again you need to know or have a fairly close estimate of velocity,length and caliber of the projectile to calculate.
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Unread postAuthor: far_cry » Wed Jul 18, 2012 5:55 am

frontstuffer wrote:Twist rate depends on velocity and length and caliber of projectile. Without knowing these there is no way to calculate twist. Most small caliber air rifles are around 1 in 16" rifling at around 700 fps. Longer projectiles need a faster twist than shorter. Twist rate is not weight dependent. Also round projectiles such as lead round balls use a slower twist rate than elongated projectiles, but again you need to know or have a fairly close estimate of velocity,length and caliber of the projectile to calculate.


wow thats a lot of info for your first post , thanks you very much

will the ammo i want to use is 8mm plastic slugs something like the 9mm hand gun bullet. 8 mm diameter and about 10-11 mm in length .
the fps is depends on the pressure and valve i use, i am shooting for
800-1000 fps ,so what twist rate is recommended?

thanks in advance
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 18, 2012 6:23 am

Enter the parameters in the calculator Gippeto linked to: http://kwk.us/twist.html

For the bullet you described made out of 1.3 g/cm<sup>3</sup> density plastic and travelling at 900 fps, suggested twist rate is about once every 8 inches.
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Unread postAuthor: al-xg » Wed Jul 18, 2012 11:04 am

I've found Miller's stability formula to be more appropriate for lighter and slower projectiles. Probably more for general interest, as either way I can't see the accuracy getting that much better just from the twist rate, there is margin for error and many other factors that will probably have a greater effect.

Can't find one that gives the twist rate as an output, but here is an online calculator: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
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Unread postAuthor: frontstuffer » Wed Jul 18, 2012 11:59 am

Correct twist rate is paramount to accuracy. 1in 8" twist sounds pretty fast for a projectile that light weight and for that length. I`m working on a rifled .32 barrel with a 1 in 16" twist and 700 fps and am getting a length of projectile of .538. I`m going at this from the other direction of finding the right bullet length for the existing twist rate I have in the .32 barrel. This is an air rifle build I`m working on.
I built a 20 ga. air shotgun with choked barrel that works very well but all I needed to do with that one was find the right weight of load and wadding for it. A rifle is a whole different critter. Have built quite a few successful muzzle loaders but the air rifles are a totally different learning curve. Still the basic premise is the same
I have zero experience with plastic projectiles but if it were me I think I would check out spiral rifled paintball barrels. They are not real expensive and may come in differing rates of twist.
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Unread postAuthor: far_cry » Sat Jul 21, 2012 2:11 am

i will be doing 1:40cm twist rate , hopefully it will give me a good accuracy
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Unread postAuthor: CS » Sat Jul 21, 2012 4:27 am

I could be barking up the wrong tree...

Sine bar rifiling machine

Since the twist rate is adjustable, you can cut and try for verifiable results.
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Unread postAuthor: frontstuffer » Sat Jul 21, 2012 9:42 am

1 in 40 cm is pretty close to 1 in 16" twist which sounds a lot closer to what will work. I would be interested in the results of your project. Good luck and have lots of fun.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 21, 2012 10:55 am

Hey there everybody! It's been a long time since I've been on the forums. Seems like everything's running as smooth as usual. :D

Back on topic, 1 in 40cm runs a stable 1.352 in the Miller stability scale which is on the low end of being stable at 900fps at 1 gram. I'd say it's about perfect. I would love to see the end result compared to a smooth bore. Should be interesting!
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