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All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: Airgun-hobbyist on November 17, 2022, 01:27:55 PM
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Can we get a discussion going on optimum twist rates for big bores... A bit of information is needed to order a few barrel blanks in different calibers (with the optimal twist rates) from TJ's. Because we are dealing with heavier slugs, both cast and swaged, what would be the ideal twist rate nowadays given the higher pressures and speeds up around 950 fps for both a 357 and a 457? TJ's offer a decent amount of options in rifle barrel liners for the 457 twist rate (getting a 45-70 liner), and only two options in 357 twist rates (one being an airgun liner).
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The "ideal" twist rate depends on the length of the slug you are shooting.... The worst stability occurs right at the speed of sound.... I use the Kolbe Twist Calculator....
http://www.geoffrey-kolbe.com/barrel_twist.htm (http://www.geoffrey-kolbe.com/barrel_twist.htm)
Input the dimensions for the longest, heaviest slug you will be using, and the output is in the form of a graph, with the twist plotted against the velocity, for a Stability Factor of 1.5, which is considered ideal.... Shorter slugs will have more stability, but that is not a problem up to about 3.0 or so.... If the Stability Factor is up around 4 or 5, and the bullets are not perfectly in balance (and they never are), the bullet may not perform properly, particularly at greater ranges.... Here is a chart for a 180 gr. in .357 cal. that is 2 calibers long with a 60% Meplat and a 2R tangent Ogive....
(https://hosting.photobucket.com/images/oo221/rsterne/.357_180_gr._Stability.jpg?width=undefined&height=undefined&fit=bounds) (https://hosting.photobucket.com/images/oo221/rsterne/.357_180_gr._Stability.jpg?width=1920&height=1080&fit=bounds)
You will note that a 26" twist is ideal (Stability factor 1.5) at 1000 fps.... It is unlikely you will be using a longer heavier bullet than that in a .357 airgun, so I would use the TJ's 26" twist as it is the slowest you can get....
For a .457 cal slug of the same shape (which would weigh about 380 gr.), you would get the same chart with a 33" twist.... so I would use the slowest twist barrel you can get from TJ's, which is a 24".... Using that twist rate, I got the following results....
(https://hosting.photobucket.com/images/oo221/rsterne/.457_380_gr._Stability.jpg?width=undefined&height=undefined&fit=bounds) (https://hosting.photobucket.com/images/oo221/rsterne/.457_380_gr._Stability.jpg?width=1920&height=1080&fit=bounds)
This is a faster twist than ideal, but at 1000 fps the Stability Factor is 2.9, which should still work just fine.... If you are using shorter slugs, then you will have more stablity than shown above, so slower twist rates can be used.... Whether or not the slugs will be spinning too fast for best accuracy depends on how fast, and the manufacturing quality of the slugs....
Bob
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Thank you for the detailed write-up Bob. In the past I had inputted a bunch of my data into a couple of those twist rate calculators but did not know if I was inputting the correct data. For example, on two different style NSA's 357 swaged slugs (both142 grains) I have two different lengths of where the rifling touched the shank of the slug, though the overall length is the same. From what I understand in your write-up they already make the ideal twist rate barrels, and looking to get the slugs spinning faster may not be optimal.
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The 26" in .357 cal is great, although you could go slower for short, light slugs.... A slower twist would be better in .457 cal, about 32" for a 400 gr., slower for the shorter, lighter stuff.... At the time I designed the .30 and .357 cal airgun barrels, a 26" twist was the slowest mandrel that TJ's could have made....
Bob
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The 26" in .357 cal is great, although you could go slower for short, light slugs.... A slower twist would be better in .457 cal, about 32" for a 400 gr., slower for the shorter, lighter stuff.... At the time I designed the .30 and .357 cal airgun barrels, a 26" twist was the slowest mandrel that TJ's could have made....
Bob
If 1/26'' in the slowest, What is the fastest twist rate you can have in .357 and still be accurate or stable with a 142gr. NSA?
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No idea.... Run the dimensions through the Kolbe Twist Calculator, and unless the SF is over 4 it will probably be fine.... Easier yet, ask Nick.... From a review on his website for that slug....
NSA 142gr perfect match for TJ 1:26
Mike Dowling on Feb 13, 2021
These are some of my favorite slugs. I shoot them just over 1000fps at the muzzle out of a re-barreled custom Bulldog. They are very accurate when combined with the TJ 1:26 barrels and higher velocities.
Bob
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The AEA big bores come with a slow twist rate, my .357 is 1-32" as is my .457.