[DudleyDR]

aka "spin rate". Part of my early education included High School Junior R.O.T.C. There was a lot of memory work involved. Some parts of it came easier to me than other parts. One question that we had to answer accurately was: "What was the purpose of rifling in a barrel?" Stock answer, "To give the bullet speed, accuracy, and stability in flight." My interest in cars started while I was in Junior High and I was very familiar with engines and RPM (Revolutions Per Minute). I was a big fan of car and gun magazines and one day back then I ran across this formula for the RPM of a bullet. Hmmm - never thought about that. I knew it was a fact, but what could it be? Not much, I presumed. Well...let's see:

M.V. x (12 / twist rate in inches) x 60 = Bullet RPM

or: M.V. x 720 divided by the twist rate in inches = Bullet RPM

Example #1: for a bullet travelling @ 2,800fps in a 1:12 twist barrel: 2,800 x 720 divided by 12 = 168,000 RPM!

Example #2: for a bullet travelling @ 3,000fps in a 1:8 twist barrel: 3,000 x 720 divided by 8 = 270,000 RPM!


I was amazed. Bullets have to be made tough. I tried to imagine a hollowpoint spinning that fast as it expands and cuts through a game animal. The thought of that dynamic spin rate hasn't left this tired old brain to this day. Muzzle velocity isn't the only physical characteristic that produces an accurate delivery downrange. The gyroscopic stabilization effect of a high speed spin helps. Velocity and twist rate are co-related and can be matched or specified for the performance one wants. There are also transonic effects and barrel heat and friction to consider. So no wonder you may find some bullet jacket residue in your barrel. There is a lot of "little stuff" that is important to what we do. I'm sure this is not news to many of you.

 

[Stevie Ray]

Facinating stuff isn't it? And if you break it down even further you'll find the average .308 round turning something around 1,500-1,800 revolutions during a little more than a half of a second in 500 yards.

 

[swagbrdr]

Yep, interesting for sure. Some of those 4000 fps varmit guns can spin the copper right off.

 

[Sniper_M-14]

I getting dizzy now got to lay down!

 

[lysander]

One question that we had to answer accurately was: "What was the purpose of rifling in a barrel?" Stock answer, "To give the bullet speed, accuracy, and stability in flight."

It doesn't give the bullet any speed.

If anything it slows it down a tad as some of the energy from the burning propellant is used up spinning the projectile, whereas it could have been used accelerating the projectile.

And, accuracy is a byproduct of being stable, but stability does not automatically equate to accuracy. Powered fin-stabilized rockets are stable, but can be quite inaccurate for various reasons.

So, the correct stock answer should be: "To stabilize the projectile."

A few things many people don't understand:

- There are two types of stability, static stability and dynamic stability, and the relationship between the two and accuracy is not cut and dry, a statically stable, but dynamically neutral projectile can be very accurate.

- A bullet is either statically stable, or it is not. A faster twist, more RPM, does not make a projectile "more stable."

- "Over-spinning" or "over-stabilizing" a projectile is largely a non-issue at in small arms, as the launch angles are too small for tractability problems

 

[DudleyDR]

It doesn't give the bullet any speed.

If anything it slows it down a tad as some of the energy from the burning propellant is used up spinning the projectile, whereas it could have been used accelerating the projectile.

Are you saying that rifling does not "give" the bullet speed by holding / retaining its speed / velocity longer at extended ranges compared to a bullet fired from a smoothbore weapon? That is the point the instructors were making. Of course anyone should know that something that causes drag by impinging / engraving rifling groves in a solid object would slow it down and not "give" it speed. That was not the point. There is no "free lunch" when it comes to energy and friction. I've never seen a magic bullet. My point was simply that I would not have guessed that a "normal" rifle bullet would have spun that fast. That is all. And I was educated.

And, accuracy is a byproduct of being stable, but stability does not automatically equate to accuracy.

Of course not and I wasn't aware that I implied that. I apologize if I left that impression with anyone. That was not my intention.

Powered fin-stabilized rockets are stable, but can be quite inaccurate for various reasons.

A given

So, the correct stock answer should be: "To stabilize the projectile."

The correct stock answer is the one that the instructors would accept and grade as correct on a test. Not necessarily the absolutely true and correct answer.

A few things many people don't understand:

- There are two types of stability, static stability and dynamic stability, and the relationship between the two and accuracy is not cut and dry, a statically stable, but dynamically neutral projectile can be very accurate.

Very true.

- A bullet is either statically stable, or it is not. A faster twist, more RPM, does not make a projectile "more stable."

Very true.

- "Over-spinning" or "over-stabilizing" a projectile is largely a non-issue at in small arms, as the launch angles are too small for tractability problems

Very True.


Respectfully submitted for clarification.

 

[2336USMC]

A couple of points to add for discussion...

I believe small arms bullets can be overspun. Any irregularity in the bullet will be exaggerated by faster spin. This has two negative effects that we can observe:

1. Reduced accuracy. There is no perfect bullet, all have flaws more or less depending on quality. The faster spin will cause the bullet to yaw more, and will deflect from its original path more before it "settles down" or achieves yaw of repose. This causes more dispersion, or larger group size. The DoD acknowledged this at the adoption of the 1/7 twist in 5.56, the TMs state that 62 gr bullets are standard with the faster twist, and that 55gr should only be used if no 62 gr is available and will show reduced accuracy. I have personally seen this effect comparing M193 55gr FMJ from 1/12 twist vs. 1/7 twist barrels, the 1/7 all showed noticeably larger groups than 1/12. I have also seen it using a .22 LR adapter in ARs, 1/12 twist barrels show pretty good accuracy, good enough for plinking and harassing ground squirrels, but using the same adapter and .22 ammo from a 1/7 twist barrel the bullets go all over the side of the barn. Way overspun for the bullet design.

2. Reduced ballistic coefficient. Again, probably due to the faster spin rate causing increased wobble in flight, the BC of a bullet goes down as the spin rate increases. David Tubb demonstrated this during design of one of his 6mm cartridges. In an effort to find the best twist rate, he built identical rifles but with slower and faster twist barrels, handloaded cartridges to give the same MV in each rifle, then fired them through chronograph screens at 1000 yds. While the MV was the same, the bullets from the slower twist barrel showed higher retained velocity at 1000 yds, therefore higher BC with the same bullet. BTW I had this discussion with the shop manager at Krieger Barrels a few years back when I was running the group buy, and he said that was absolutely true.

This leads me to the conclusion that we should consider using the slowest twist that will adequately stabilize the bullets we want to use. If you really want a faster twist barrel, use high quality match grade bullets and just accept a slight loss of BC that most of us wouldn't really see in real life unless we measured it... but every little bit helps.

 

[lysander]

[1]Are you saying that rifling does not "give" the bullet speed by holding its speed / velocity longer at extended ranges compared to a bullet fired from a smoothbore weapon? That is the point the instructors were making. Of course anyone should know that something that causes drag by impinging / engraving rifling groves in a solid object would slow it down and not "give" it speed. That was not the point. There is no "free lunch" when it comes to energy and friction. I've never seen a magic bullet. My point was simply that I would not have guessed that a "normal" rifle bullet would have spun that fast. That is all. And I was educated.

And, accuracy is a byproduct of being stable, but stability does not automatically equate to accuracy.

Of course not and I wasn't aware that I implied that. I apologize if I left that impression with anyone. That was not my intention.

Powered fin-stabilized rockets are stable, but can be quite inaccurate for various reasons.

[2]A given

So, the correct stock answer should be: "To stabilize the projectile."

[3]The correct stock answer is the one that the instructors would accept and grade as correct on a test. Not necessarily the absolutely true and correct answer.

Respectfully submitted for clarification.

[1] A fin-stabilized projectile retains velocity down range just as well and a spun projectile. Depending on the design, it can retain more, see a fin-stabilized discarding sabot AT projectiles, they have absurdly high BCs, and are really damn accurate.

[2] Not for the reasons you think. it has to do with the fact they are powered, the effects of cross-winds on the tail fins, and the position of the relative wind with respect to the line-of-sight to the intended target.

[3] This was not a reflection on you, but the rather poor, and somewhat over-simplified explanation of exterior ballistics given, in the past and even today. Note, I stated "should be"....

 

[Duce]

9mm pistol bullet spinning...

[ame]https://www.youtube.com/watch?v=WW7OCcrhdNU[/ame]

40S&W pistol bullet spinning...

[ame]https://www.youtube.com/watch?v=foZlciP6gUQ[/ame]

7.62 Bullet spinning

[ame]https://www.youtube.com/watch?v=UwIpv94GPt4[/ame]

 

[lysander]

A couple of points to add for discussion...

I believe small arms bullets can be overspun. Any irregularity in the bullet will be exaggerated by faster spin. This has two negative effects that we can observe:

1. Reduced accuracy. There is no perfect bullet, all have flaws more or less depending on quality. The faster spin will cause the bullet to yaw more, and will deflect from its original path more before it "settles down" or achieves yaw of repose. This causes more dispersion, or larger group size. The DoD acknowledged this at the adoption of the 1/7 twist in 5.56, the TMs state that 62 gr bullets are standard with the faster twist, and that 55gr should only be used if no 62 gr is available and will show reduced accuracy. I have personally seen this effect comparing M193 55gr FMJ from 1/12 twist vs. 1/7 twist barrels, the 1/7 all showed noticeably larger groups than 1/12. I have also seen it using a .22 LR adapter in ARs, 1/12 twist barrels show pretty good accuracy, good enough for plinking and harassing ground squirrels, but using the same adapter and .22 ammo from a 1/7 twist barrel the bullets go all over the side of the barn. Way overspun for the bullet design.

2. Reduced ballistic coefficient. Again, probably due to the faster spin rate causing increased wobble in flight, the BC of a bullet goes down as the spin rate increases. David Tubb demonstrated this during design of one of his 6mm cartridges. In an effort to find the best twist rate, he built identical rifles but with slower and faster twist barrels, handloaded cartridges to give the same MV in each rifle, then fired them through chronograph screens at 1000 yds. While the MV was the same, the bullets from the slower twist barrel showed higher retained velocity at 1000 yds, therefore higher BC with the same bullet. BTW I had this discussion with the shop manager at Krieger Barrels a few years back when I was running the group buy, and he said that was absolutely true.

This leads me to the conclusion that we should consider using the slowest twist that will adequately stabilize the bullets we want to use. If you really want a faster twist barrel, use high quality match grade bullets and just accept a slight loss of BC that most of us wouldn't really see in real life unless we measured it... but every little bit helps.

Whether or not, high spin rates effect BC depends on the "other stability factor", the dynamic stability, of the projectile. A projectile with a high coefficient of dynamic stability will damp out its yaw faster than one with a neutral or lower coefficient of dynamic stability. Once the initial yaw is damped out and the yaw of repose is established, minimum drag conditions exist. In some cases, a high spin can make for a higher dynamic stability factor.

Basically, it all depends on the projectile design. Some bullets may like some twists better than others, but it is difficult to generalize with any degree of accuracy.

Projectile wobble in free flight does not effect accuracy, other than making the point of impact a circle the diameter of twice the offset of the actual CG to the geometric centerline of the projectile. The actual effect on accuracy is caused at the instant it uncorks from the barrel. As the projectile travels down the bore it is constrained to rotate about the center of the bore. Once free of this constraint, the projectile will "jump" to a new center of rotation about the CG. Once the "jump" is complete, there are no other lateral forces on the projectile (ignoring the usual cross wind, drag and gravity, etc, which will always be present even with a "perfect bullet"). However, since there are no other forces acting on the projectile, it will continue to drift in the direction of the initial jump. The magnitude of the jump and the resulting lateral velocity is dependent on the angular velocity and the magnitude of the eccentricity of the CG to the geometric centerline. With a good grade of match bullets, the overall effect is lost in the general variation of Vm, wind, drag, and other factors effecting projectile flight.

Others have found little difference in accuracy ("technically precision" to quote the author...) http://www.m4carbine.net/showthread...Fired-From-AR-15s-with-1-7-quot-Twist-Barrels

 

[smoothy8500]

Oh no, we're not going to re-visit the arguments over the theories of cannons, recoil, and relativity again are we?

 

[DudleyDR]

Oh no, we're not going to re-visit the arguments over the theories of cannons, recoil, and relativity again are we?

Well no, I'm not. Just wanted to recall a time of awareness for this young pup. Reminiscing can be "dangerous". I apologize.

 

[Earthquake]

I know that 40 gn .22 caliber bullets designed for the .22 Hornet coming out of a .22-250 at 3800+ FPS don't go more then about 5 feet out of the barrel before they blow. Its like a hi speed mini shotgun.

Casey

 

[nortac]

One thing to remember with these high RPM numbers is, the bullet is not (usually) spinning for a whole minute. And when it gets to its target, particularly a soft target into which it comes to rest and does not exit, it doesn't sit there spinning away. I remember a discussion re: the Black Talon pistol bullets, where a ballistically naive anti gun person was horrified that the expanded Talon was like a buzz saw spinning at many thousand RPM. But fired into ballistic gelatin, it is clearly seen that it only turned a couple of revolutions before it came to rest in the material, as evidenced by the spiral tracks cut by the "talons" in the wound channel. The above ice video is obviously a special case.

 

[MitchAlsup]

Example #1: for a bullet travelling @ 2,800fps in a 1:12 twist barrel: 2,800 x 720 divided by 12 = 168,000 RPM!

Example #2: for a bullet travelling @ 3,000fps in a 1:8 twist barrel: 3,000 x 720 divided by 8 = 270,000 RPM!

Now consider the speed at the edge of the bullet:

A 308 bullet is <well> 0.308" in diameter 0.9676" around its perimeter.
A 223 bullet is 0.224" in diameter and 0.7037" around its perimeter.

Thus, the speed of the 308 at its edge is 162,558 "/s
Thus, the speed of the 223 at its edge is 190,003 "/s

Or almost the same speed at the edge!

 

[Ted Brown]

I always wondered what those funny looking twisty things inside the barrel were for. They spin the them bullets real fast! Beer make my head spin like that...

 

[DudleyDR]

I always wondered what those funny looking twisty things inside the barrel were for. They spin the them bullets real fast! Beer make my head spin like that...

Looking at them can make a person dizzy. I think they were put there by the folks who make bore cleaning products just to make life difficult.

 

[Samuel Seeley]

I tried to imagine a hollowpoint spinning that fast as it expands and cuts through a game animal.

Think about it this way:
1) If your barrel has a 1 in 10" twist, and
2) Your bullet penetrates 10" in a game animal, then
3) The bullet decelerates to zero in 10 inches.
4) The rotation also goes to zero in 10" (only completes one turn)

What's the rate of deceleration? DI2

 

[DudleyDR]

Think about it this way:
1) If your barrel has a 1 in 10" twist, and
2) Your bullet penetrates 10" in a game animal, then
3) The bullet decelerates to zero in 10 inches.
4) The rotation also goes to zero in 10" (only completes one turn)

What's the rate of deceleration? DI2

"...(only completes one turn)"

False. Bad thinking. You can't ignore the necessary obvious facts in your calculation. That is "static, linear" thinking as if you were pushing a bullet 10" down a barrel with a cleaning rod w/o considering the velocity and RPM when the bullet entered the target, its caliber / cross section, bullet weight and design, expansion, petal deployment, fragmentation, etc. Think dynamically. In 3-D, and in motion, if you will. You will need more data of a specific nature and a calculus equation to solve your scenario - and then it will be only an estimate.

Assuming that the bullet is travelling at 2,800fps from a barrel with a 1:10" twist when it enters this target of unknown material, composition, and consistency, it will spin 2,016 times in the first 1/100 of an inch.

 

[loader21]