Bill_Rights

November 15, 2012, 10:54 PM

We have at least three threads going on now/recently that raise the question of rifling twist rate versus bullet weight. Here they are:

Thoughts on fast twist 6.8 SPC barrel

http://www.thehighroad.org/showthread.php?t=685374

180gr in a .270 Winchester?

http://www.thehighroad.org/showthread.php?t=685010

Rossi 92

http://www.thehighroad.org/showthread.php?t=684296

What determines the minimum twist rate of a barrel? [For "low" twist rate, I would cite 1 turn in 30 inches, as some old big-bore black powder rifles have. For "high" twist rate, I would cite 1 turn in 7 inches, as some 5.56mm/.223" short-barrel rifles have.]

How does twist rate play in? I have picked up that, if twist rate is not high enough, the bullet will not "stabilize". [By the way, what do we mean "not stabilize"?] Is that true? (I assume this is true, in extreme cases.)

But I am seeing people post here on THR that for heavy bullets, you need a higher twist rate than with a light bullet of the same caliber. How's that?

What is the total picture? What is the theory? I also picked up that smaller calibers have higher twist rate while larger calibers have lower twist rate. Why?

So it looks like desirable or ideal twist rate depends upon both bullet mass and bullet diameter. Does it also depend upon bullet speed? (expected muzzle velocity?) That is the same as saying that it depends on powder loading of the cartridge. right?

I know that the absolute spin rate of the flying bullet (rpm, revolutions per minute) is faster the faster the bullet travels. For example, for a 1-in-12" twist rate, a bullet exiting the muzzle at 2000 fps would be spinning at 2000 rotations per second = 2000 rps * 60 sec/min = 120,000 rpm. For the same 1-in-12" twist rate, a bullet exiting the muzzle at 3000 fps would be spinning at 3000 rotations per second = 3000 rps * 60 sec/min = 180,000 rpm.

I would guess that what matters for in-flight ballistics is the spin rate of the flying bullet, not the twist rate of the barrel, right?

So what is the formula where bullet mass, bullet diameter and bullet speed factor into the desirable spin speed or twist rate?

Are there any other factors? I have also picked up here on THR that length of the bullet matters. True? Or is it sectional density or other measurement?

HELP! I am in the dark here. :confused:

Thoughts on fast twist 6.8 SPC barrel

http://www.thehighroad.org/showthread.php?t=685374

180gr in a .270 Winchester?

http://www.thehighroad.org/showthread.php?t=685010

Rossi 92

http://www.thehighroad.org/showthread.php?t=684296

What determines the minimum twist rate of a barrel? [For "low" twist rate, I would cite 1 turn in 30 inches, as some old big-bore black powder rifles have. For "high" twist rate, I would cite 1 turn in 7 inches, as some 5.56mm/.223" short-barrel rifles have.]

How does twist rate play in? I have picked up that, if twist rate is not high enough, the bullet will not "stabilize". [By the way, what do we mean "not stabilize"?] Is that true? (I assume this is true, in extreme cases.)

But I am seeing people post here on THR that for heavy bullets, you need a higher twist rate than with a light bullet of the same caliber. How's that?

What is the total picture? What is the theory? I also picked up that smaller calibers have higher twist rate while larger calibers have lower twist rate. Why?

So it looks like desirable or ideal twist rate depends upon both bullet mass and bullet diameter. Does it also depend upon bullet speed? (expected muzzle velocity?) That is the same as saying that it depends on powder loading of the cartridge. right?

I know that the absolute spin rate of the flying bullet (rpm, revolutions per minute) is faster the faster the bullet travels. For example, for a 1-in-12" twist rate, a bullet exiting the muzzle at 2000 fps would be spinning at 2000 rotations per second = 2000 rps * 60 sec/min = 120,000 rpm. For the same 1-in-12" twist rate, a bullet exiting the muzzle at 3000 fps would be spinning at 3000 rotations per second = 3000 rps * 60 sec/min = 180,000 rpm.

I would guess that what matters for in-flight ballistics is the spin rate of the flying bullet, not the twist rate of the barrel, right?

So what is the formula where bullet mass, bullet diameter and bullet speed factor into the desirable spin speed or twist rate?

Are there any other factors? I have also picked up here on THR that length of the bullet matters. True? Or is it sectional density or other measurement?

HELP! I am in the dark here. :confused: