Author Topic: Twist rate vs weight vs length  (Read 262 times)

Merle

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Twist rate vs weight vs length
« on: October 23, 2020, 02:41:44 PM »
Hello, I have Googled, hit so many forums on face book and driven everyone who will listen absolutely crazy trying to figure this out and out of outright desperation finally posted here. My question is (please keep in mind I'm very new to this)  I have a Benjamin Bulldog that's being power tuned to a 250 ft lb gun, to my understanding the Bulldog has a 1:16 twist. So the general consensus is it likes "heavier" lead. Is it really the weight that comes into play or does the length part of the equation as well? I've been told that it will be a 250 ft lb gun with 150gr ammo...
The upgrades will be widened transfer port, balanced valve, 3500 psi tank and whatever other magic and wizardry my Smith does to obtain that 250 ft lbs. I have found several 145gr. To 165gr slugs that run .73 to .75 Oal which is the largest I can fit in the magazine. With that in mind... Am I looking in the right direction? This will be a hunting rifle- coyote mostly but I don't want to rule out deer hopefully. And can I milk more ft lbs out of it by using a heavier slug even though it's been tuned for that 150gr range?
Any and all input would be greatly welcomed.


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rsterne

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Re: Twist rate vs weight vs length
« Reply #1 on: October 23, 2020, 02:54:18 PM »
With a 16" twist in .357 cal. it doesn't surprise me that it has a reputation as liking "heavier" ammo.... Heavier slugs tend to be longer, and you are quite correct, it is primarily the length that affects the optimum twist rate.... It is unlikely you could use a long enough bullet to get to optimum for that twist rate, something like a 210 gr. boattail would be required, and that is much too heavy for the FPE you will have.... For bullets in the 150 gr. range, you would be better off with a much slower twist of 22-26"....

Spinning a bullet faster than required causes two problems.... First, it is more susceptible to "Aerodynamic Jump", which is the vertical deflection you get in a crosswind.... This causes groups in gusty conditions to be angled, rather than purely horizontal (from the gusts).... Secondly, an overspun bullet is more susceptible to Dynamic Instability (increasing yaw angles with distance), which can lead to spiralling (corkscrewing) and lousy groups....

Having said that, bullets and guns can't read, and are little affected by theory.... As with any airgun, try several slugs until you find one that your gun likes....  ;)

Bob

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Merle

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Re: Twist rate vs weight vs length
« Reply #2 on: October 23, 2020, 03:12:34 PM »
Much thanks! That's the most direct answer I've gotten so far...before the tune I was shooting 110gr nsa slugs with good results. I'm hoping with the tune I can get tighter groups. But could you please address the second part of my original question (I wasn't kidding when I used the term "wizardry" it's all magic to me lol) being tuned to the 150gr range can I expect poorer results from heavier or for that matter lighter slugs?   I realize that it's very subjective with many other factors coming into play, I'm just attempting to get a fair starting point as well as educate myself a little.
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rsterne

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Re: Twist rate vs weight vs length
« Reply #3 on: October 23, 2020, 09:46:24 PM »
The "tune" of the gun, while optimizing it velocity-wise for a certain weight of slug, may not have any bearing on what ends up the most accurate.... Having said that, if you want to stay close to 150 gr., try and get as many designs as you can to find the most accurate choice for your gun....

Bob
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