OK now I have a question. I ran 2 different bullets through the above calculators and I came up with numbers of 4.6 and 6.3. According to this calculation my bullet is over stabilized! What does that mean and how does it effect accuracy?
Now that's a heavy question, bronco. In prior times, a lot of concern was given to a bullet being "over stabilized", meaning spun faster than it really needed to be. This has not proven to be much of a problem, to my knowledge. The old Greenhill formula for twist rate and thus bullet stabilization has not held up well over the years as bullets got better and better and also slimmer, and longer.
It can get very complicated because as a fired bullet slows and nears the sonic barrier, the speed of sound at a bit over 1100 fps depending on altitude and other factors, it becomes most stable.
Let's look at some examples: Pedersoli built its .45 Mortimer-Whitworth competition ML rifles with a 1:20.7 twist rate (call it 21, as many do) because it was designed specifically to shoot a long-shank, multi-groove lubed bullet with moderate ogive (nose curve) plus moderate meplat (flat) front for international competition at 100 meters. The bullet is 500 grains wt., .449 D. and 1.24" long. No doubt Pedersoli was aiming for it to be shot very close to the speed of sound or possibly exceed this a bit, but be slowing to almost the speed of sound as it reached 100 meters. What you don't want is for the bullet to be yawing a bit as it becomes somewhat unstable passing thru the sound barrier. For a large and relatively low B.C. bullet like a muzzleloader's often still are, if you want great accuracy at 100 yds. (or 110 yds., which is 100 meters) shoot it below the speed of sound, but not by much.
Now let's look at your "over stabilized" bullet. For a ML like the Pedersoli above, the shorter 500 gr.bullet is perfect for 100 meters, and works pretty well for 200 too. But for long range, like 600-1000 yards, you need a longer bullet of same .45 cal. (considered best cal. for this by many) and this requires more spin. Bullet is again .449 D. but now is about 545 gr. and about 1.4" long as I recall. So Pedersoli built the now-famous "Gibbs" which is just the Mortimer-Witworth with a 1:18 twist barrel instead of 1:21. That would "over spin" the shorter bullet for 100 meter shooting, but not by much. Nonetheless, these little things is what wins Gold Medals, and that's what Pedersoli is good at.
So you will likely find that your "overspin" is a lot better than "underspin" and that's why a .50 or .54 T/C Renegade at 1:48 twist will shoot RBs well (tho "overspun", since 1:72 twist would be better for RBs in those calbers, and is pretty good for bore-sized slugs or 45 bullets in sabots as a "compromise twist", tho 1:28 would be better with the usual velocities.
Lastly, to really cloud the picture. You can "underspin" or "overspin" your ML bullet with a given cal. and twist rate, depending on its length and also its velocity. If you have an "overspin" twist rate, just use
less powder and bingo the
rps (revolutions per second) at the muzzle will be less.
Give it
more powder if you have an "underspin" twist rate and thus gain the rps provided more easily by a faster twist rate, to shoot even to 1000 yards if circumstances match up. That's how I could shoot my 1:21 twist Pedersoli Mortimer well to 1000 yds even tho I used the longer 545 gr. bullets. I simply used 107 gr. by wt. of Swiss 1.5F, instead of the usual 85-95 gr. That made for significant recoil, but spun my bullet like 1:18 twist does with the lesser charges. My higher-veocity bullet was leaving my muzzle at very close to the same rps as the bullets from 1:18 twist rifles!
Moral of the story: it is
rps at the muzzle, not nominal twist rate, that determines whether the usual ML bullet will be and remain stable, especially at longer ranges, assuming a bullet of proper length is being used. Oh, how often I see bullet flight and accuracy discussion as if twist rate is what stablizes a bullet. No....velocity coupled with bullet length must be part of the equation. But notably, for MLs, B.C. (ballistic coefficient") is of little importance at 100 or 200 yds. (It's a different story when you get into modern muzzleloaders shooting at 2000-3000 fps and using narrower and long modern bullets in sabots. B.C. gets more and more important, especially at the ranges beyond 200 yds. or so.)
And here's a real mind-blower: bullets oscillate after suddenly leaving the muzzle, then "settle down" from this as they get farther from the muzzle. So they may be moderately accurate at 100 yds., yet be more accurate at 200!! I thought this was BS for along time, but Dr. Franklin Mann proved this back in 1902 by careful experiment. His book, "The Bullet's Flight from Powder to Target is a marvel, and has been reprinted.
Aloha, Ka'imiloa
