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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Barrel Twist Rates for .177, .20, .22 & .25 caliber Slugs

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SpiralGroove:
Hey Bob,
I'm shooting slugs out of my RAW HM1000x PCP with the above caliber barrels.

If I go to the Barrel Outlet's Twist Rate calculator ... all the above slug calibers require more than a 1-11" twist rate at 950 fps?

Yet the RAW airgun comes with a 1-17.7" LW/Poly barrel and seems to shoot well at 100 yards.

What is marginal problem with this finding or my barrels given the (GS) model used?

Hence, the ideal or preferred rate should be much higher for slug shooting?

Any thoughts are appreciated ;).
Kirk

rsterne:
By higher, do you mean faster (the number in inches is smaller), or a larger number (in inches), so a slower twist rate?.... It would help if you could link to "Barrel Outlet's" twist rate calculator.... Some are designed only for PB's and Supersonic, a few (very few) are intended to be used for Subsonic and Transonic, which is a different kettle of fish....

OK, so I Googled it, and it uses the "Don Miller Twist Rule", which is definitely not intended for lower velocities.... In fact, at 1200 fps and under, it gives a warning that says "Muzzle Velocity LOW!!".... If you keep putting in lower and lower velocities, the suggested twist rate keeps getting faster and faster (wrong!).... The example I tried was a .25 cal that is 3 calibers (0.75") long, and at 1500 fps it suggests a twist of 10.8", but at 1200 it says 9.7", at 900 is comes up with 8.4" and at 600 just 6.8".... If I use the Kolbe Twist Calculator, which DOES provide for lower velocities, for the same LOA and caliber, with a flat base (the Miller Rule does not allow for boattails anyways) and a half-caliber long nose and 50% Meplat (typical for an airgun slug), I get the following for a GS (Gyroscopic Stability, also known as SG) of 1.5 (recommended)....



I use either the Kolbe or JBM Twist Calculators exclusively, as they are the only one I have found based on the work of Robert McCoy while he was working at the Aberdeen Proving Grounds, and are suitable for subsonic use.... There may be others I am not aware of....

http://www.geoffrey-kolbe.com/barrel_twist.htm

https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

You don't give me the length slugs you are shooting, but it is entirely possible that 17.7" is fast enough, depending on calibre.... For a .25 cal slug that is 0.50" long, 17.7" is actually faster than needed!.... Twist is actually calculated in "calibres", so for a given shape, a smaller slug will need a faster twist.... I have no idea why LW uses the same twist for all their airgun (pellet) barrels.... Current thinking on pellet barrels is that they should be between 100-200 calibres twist rate.... so for a .25 cal from 25-50" twist....

Bob

SpiralGroove:
Bob,
Thanks for clearing that up ;).
-> Yes, I meant as you noted, the website recommended more twist per inch than at lower airgun MV's.

Sorry :P,
I forgot to you the link for Barrel  Outlets calculator ......

ballisticboy:
Even the Kolbe calculator is inaccurate at speeds between about 1000 and 1200 ft/sec. Bob McCoy's equations were really for rifle bullets fired at supersonic speeds, and appear to be very simplified versions based on the form of the equations used in Intlift. The equations he created are not accurate as speeds approach Mach 1 and were, I believe, never intended for detail use at such speeds.

The other problem is that the bullet is assumed solid with no hollow points or base cavities, all of which will affect the centre of gravity position and thus the twist rate needed for stability. Intlift on the JBM site will calculate some of the parameters needed, but you would still need centre of gravity position and other things like estimated inertias, which you are not likely to be able to obtain. I do not know of any other available software which can give an estimate.

rsterne:
Thanks for that, Miles.... Bottom line, what would you use to give you the best chance of having sufficient stability for airgun slugs at 1100 fps and less, without being drastically overspun?.... I use Kolbe because I have not found anything better.... I realize it may overestimate the rate of twist required, but nowhere near as much as using the Miller Twist Rule, for example....  My Bob's Boattails seem to be able to get away with a bit slower twist than Kolbe says, for example, but I have not run into any situation where it underestimated the twist rate.... Your suggestion would be most welcome....

BTW, I usually use Kolbe to determine an SG of 1.5 at 1000 fps, and use that twist rate, because you told me before it isn't great around Mach 1....  ;)

Bob

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