Rifling twist rates
Hello I understand for any given bore size the longer the bullet the faster the twist needs to be. My question is this does the recomended twist rate stay the same on a slow moveing bullet like one in a 25/20 that weighs 86 Grains and a faster moving bullet like a 87 Grain in a 257 Weatherby Mag. or in pistols with the same bullet like one in 38 short colt and 357 HERRET.
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Interesting question, Perry---What I understand the question to be is whether the RPM of similar bullets needs to be the same for proper stability. The .257 would be about 50%+ over the 25-20, huh? You'd think that the 25-20 would have to have a 50% faster twist to keep its bullets stable, wouldn't you?
Let's see what the other guys say!!
Cheers,
OleDuk[:)]
"Disperse you Rebels; Damn you,throw down your arms and disperse!"
British Major John Pitcairn, April 19, 1775. He fell at Bunker Hill two months later.0 -
The key factor is length of projectile, not axial rotation. The formula you need to calculate correct rate of twist is: (t)(l)=150
Example:
The optimum rate of twist for Sierra's .30-caliber 200-gr MatchKing would be computed as (t)(l)=150 where:
t=rate of twist
l=bullet length in calibers (1.405"/.308) or 4.561688311688
t=150/4.56
t=32.89 calibers
To convert calibers to inches, multiply by the bullet diameter. In this example, (32.89)(.308)=10.1 inches.
The proper rate of twist for the Sierra .30-caliber, 200-gr MatchKing is 1:10". This formula will solve for any of its variables.
Be careful to convert your twist from CALIBERS to inches. Otherwise you will obtain bizarre answers. I know. I do it all the time.
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I considered trying to compensate for a too slow twist by goosing velocity. Doesn't work because excessive axial rotation caused by excessive velocity induces another inaccuracy causer. It's called the Magnus Effect. You and I know it as the principle that causes the curve ball low and away.0 -
Thanks For your input The reason I ask the question Is I want to shoot a bullet that shoots very small groups if you push it fast but when you try to slow it down The groups go to pot . I KNOW SHOOT THEM FAST. HOWEVER this is in a pistol and I am looking to lower recoil.I would like to know if changeing the barrel twist for a faster one whould give good groups at slower speeds because the "RPM" of the bullet would than be the same. My need or want is for pistols but I used the rifle as one of the examples because I thought more people experimented with rifles than pistols.None of the formulas that I have ever seen had any thing about VEL. Thanks again Karl 0 -
Lance
Greenhill Simplified (the formula you posted) works for bullets launched in the 2200-2900fps range, above that the factor should be 180, below that something on the order of 125 would work better.
Perry
Velocity does play a roll, as the rpm is a factor of twist and velocity.
Go to http://www.lostriverballistic.com/LRB/DocsCorner-Greenhill.cfm, the complete "headache math" formulia for bullet stability (for determining the spin rate that needs to be plugged into the formulia SR=MV*TW) is posted there. BTW, the auther of that page is also the designer of the 408Chey-Tac.
Some guys like a mag full of lead, I still prefer one round to the head.0
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