If I do the math, 1 in 7 twist, goes 1.4 +/- revs per foot.
3000 fps at the muzzle means at the muzzle, the bullet is going 3000fps x 1.4 revs per foot x 60 seconds,
= over 250k rpm at the muzzle.
I think that's right
That should be 1.7 revs per foot: 12 ÷ 7 = 1.71428. Here's your reality check: If it were 1 in 6" twist, it would be 2 revolutions per foot.
Most varmint rifles put their bullets out at around a quarter-million RPM. The .220 Swift was known to have its bullets fly apart from the spinning, leaving a trail of blue-ish smoke to the target. Or halfway to the target, anyhow.
It's the twist at the muzzle as the bullet exits that dictates the RPM, correct? Most modern guns has a steady twist rate from chamber to muzzle, but I've seen some older guns that utilize a progressively tighter rate of twist where it's spin is slower at the chamber and spin increases as it travels down the barrel. Was/ is there any benefit to this, and does it effect spin rate of the bullet as it exits that barrel?
Those are called "Gain Twist" rifles/barrels. Most of the time they don't work out too well because the additional engraving on the bullet distorts it too much, as well as reducing the gas seal of the bullet in the bore. The spin depends on the last rifling the bullet "saw" as it left the muzzle. The distortion wasn't that much of a problem with patched bullets or round balls.*
It is the velocity coming out of the muzzle at a given twist rate which determines the RPM. Faster muzzle velocity = faster spin.
Now as to what twist is best for a given bullet, look up "the Greenhill Formula," which is an approximation (a guideline) for the probable best twist to start with for a given bullet.
Terry, 230RN
* There's always a lot of experimenting going on, including the concept of "tapered" bores which had their diameters reduced as the bullet went down the barrel. The bullets were flanged, so the flange would collapse as the bore diameter got smaller. An example is the 14/9 mm Gerlich cartridge, where the bore went from 14mm to 9mm at the muzzle. (Slip
that one in next time there's a "9 versus .45" debate !
)
From Datig's "Cartridges for collectors: