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Rifling Twist Rate

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dcriner

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Why do barrels designed for round balls typically have slower twists than those designed for conical bullets?
 
Swampy pretty much nailed it. :thumbsup:

With an elongated bullet a fast spin is needed to create a gyroscopic effect to keep the bullet from tumbling, or turning end over end and leaving a "keyhole" shape on whatever it hits.

With a roundball the spinning from the rifling is not needed to keep the ball from tumbling but it plays an important part in keeping the ball flying in a true direction.

The surface of a roundball is not smooth and uniformly shaped although it does look pretty good to the human eye.

To the wind at subsonic and supersonic speeds it looks much different. The uneven surface will develop different pressures from the air blast and if the ball is not spinning (like it was fired from a smoothbored gun) these unequal pressures will cause the ball to be pushed off course. The direction it will fly is impossible to figure out.

If the ball is spinning about the axis of its flight these same pressures are trying to deflect the ball however because it is spinning the actual direction of deflection is always changing.

Put another way, lets say that at one position the pressure is pushing the ball to the right.
If the ball was not spinning it would start going to the right immediately after leaving the muzzle.
With it spinning, just about the time the pressure was going to push the ball to the right, it is now trying to push it downward to the right, then down, then down to the left, then to the left, then up to the left, then up, and so forth.
The deflecting pressure is always changing its direction about the axis of the balls spin and along the axis of its flight. The net effect of all of this is the ball is not really deflected at all so it continues to travel along its trajectory towards the target.

To accomplish this great task the ball does not need a super fast spin so the slower rate of twist works fine.

The slower rate of twist does another good thing for a patched ball. It does not tear up the soft cloth patch even if some very high velocities are used.
This makes a slow twist barrel much less sensitive to high or low powder loads (velocities). :)
 
dcriner said:
Why do barrels designed for round balls typically have slower twists than those designed for conical bullets?

The cloth patch will not tolerate a lot of torque.
Fast twists increase the torque applied to the patch. Too much torque and the patch shreds.
Also rifling twists are designed for a specific bullet LENGTH and design. The 58 Minie Bullet with its large hollow base will stabilize in a 72" twist, the US standard for the Rifle Musket. But the large hollow limits velocity since heavy charges can deform the skirt.
Bullets also generally work better with shallow grooves. .003-.005" deep. Barrels for cloth patched projectiles need .008-.010 to work well.

A bullet two calibers long will stabilize in a 48" twist but its not ideal.
48" works well for most round balls but so will 66" or 72" or 80 or 120 if the ball is large diameter. Bullets are not so forgiving and need a specific rate of twist for given bullet length to work best.
A 400 grain 40 caliber BULLET that is over 3 calibers long may need a 16 to 18" twist for best results.
Search the WWW for "Greenhill Formula". Its a little off 38-45 caliber projectiles at BP velocities but its still a good ball park.
This page might be of interest too.
http://kwk.us/twist.html
Gives a 77" twist for a 50 caliber RB at 2000 fps. A little slow, I would prefer 48 to 66. But it will work.
A one inch long 50 caliber bullet (estimated at .510") requires about 34" which is probably a little fast but ball park for a "picket" rifle using a cloth patched elongated bullet and would work in smaller bores as well like 38 to 40 calibers.

Dan
 
Great information, DAN. Most people, when seeking to compare Bullets to Round balls, forget, or don't understand, that a RB HAS to be shot wrapped in a cloth patch to be accurate in a rifle. The slow ROT is necessary to protect that cloth patch. The depth of the rifling is to allow that cloth patch to make a tight fit between the RB and the Grooves , so that you both have an Adequate Gas Seal, and a good way to transfer the spin of the rifling to the Round Ball.

Round Balls ARE NOT normally groove diameter, and if they were, the soft lead still cannot stand the Torgue of a fast ROT by itself. The torque will strip the lead ball on the rifling and you get no good accuracy at all. That is why lead BULLETS most often are made of alloys of lead, containing tin, and sometimes antimony, to make a BARE LEAD ALLOY BULLET harder.

The fabric patch allows us to shoot a RB that is several thousands of an inch smaller than the bore diameter( land to land). That ball will often drop right down the barrel if loaded without a cloth patch wrapped around it, in a clean barrel.

Trying to compare Round Balls to Lead bullets is like comparing apples to oranges. And, comparing the ROT needed to stabilize each of these is in the same realm of problems.

The Nice thing about getting into BP sports today is that all the work needed to find the optimum rates of twist, and groove depth for rifling a barrel to shoot either Round Balls, or bullets, has long been done for us. :bow: :v
 
Actually, the term "slow twist" is kinda misleading. The actual speed with which a ball is spinning after being shot is pretty amazing. I once saw a chart on this and the RPMs generated were sumptin' else.
 
Rifleman1776, Are you saying the ball will be spinning at a faster rate than the rate of twist in the barrel. It seems the inertial thrust upon exiting the barrel would be the same as the twist rate itself, or maybe gain a bit at the loss of barrel friction. I don't know, just asking.
Robby
 
The amount of spin varies with the velocity. Let's just say the MV is 1600 feet/second. Nice number to keep the math simple. Rate of twist is 1 in 48" or 1 turn every 4 feet, right? OK 1600 divides by 4 to yield 400 turns per second. Multiply that by 60 seconds in a minute and WOW 24,000 rpm. That is PDQ for sure. Now if you stoke your load for a faster velocity Do you see how that will increase the RPM of the ball? V/ROT(in turns per foot)times 60= RPM
 
Gerard, I understand that. Maybe I misunderstood the implication. Say the twist is 1 in 48", shooting at a target 100 yards away the ball will rotate approximately 25 times, no matter the time it takes to get there. That was my thinking.
Robby
 
If you are shooting in a vacuum, yes, this is correct. However, in air, you are always dealing with "Drag Factors", that not only affect forward movement( changes in velocity, and time in flight) but also Rotational speed. Still, even if that ball rotates only half that number of times, its still a "high speed" drill. NO? :v :grin:
 
dcriner said:
Why do barrels designed for round balls typically have slower twists than those designed for conical bullets?

The longer a projectile is in relation to it's diameter, increased spin is required to stabilize it. A round ball has very little sectional density as opposed to a conical.
 
Robby said:
Gerard, I understand that. Maybe I misunderstood the implication. Say the twist is 1 in 48", shooting at a target 100 yards away the ball will rotate approximately 25 times, no matter the time it takes to get there. That was my thinking.
Robby

That is "my thinking" also. However I (we?) apparently do not have mathematical minds. Gerard has the right approach. His actual numbers may be off but the concept is correct. A fired round ball, whether it is from a 1:48 barrel or a 1:72 one is spinning really-really fast.
 
Well, there's a few mix up of terms that can confuse us. Gerard is speaking strictly of RPM, so maybe he should have started his post by saying "The SPEED of spin varies with velocity", instead of "The amount of spin varies".

The amount, or number of times a balls spins is dictated by the distance the projectlie travels, independent of the velocity. But the number of spins has no influence of stability though, RPM is the factor that influences bullet stability, which is what I think Gerard was getting at :thumbsup: .

If you spin a top very slowly, it wobbles around and falls over pretty soon. But, if you change the RPM of the top (in this case increase the RPM), the top spins stable and won't wobble. Same thing with a projectile. It's not a matter of how many turns it completes, it's a matter of how many turns in a given period of time that is the stabalizing factor (RPM).

P.S. 100 feet will spin a 1:48 twist 25 times, not 100 yards. :grin: Bill
 
Sorry I was calling spin = RPM as opposed to rate of twist which is a constant for every gun. 75 turns in a blink is still awfully fast. When we fiddle around with the amount of powder to find our best load I believe we are adjusting the RPM to get the most stability from our projectile. :hatsoff:
 
I never thought of it that way, but yep, that's what we're doing. :thumbsup: The windage and elevation thing can be adjusted with the sights, but adjusting the powder for the smallest groups is simply tweaking the RPM. Good post. Bill
 
Adjusting velocity for a given projectile has more to do with finding the right level of barrel harmonics than in finding the right rpm.

You want that barrel vibrating the same amount and in the same place each time a ball/bullet leaves that muzzle for best accuracy. NO? :hmm:
 
I remember playing with a friend's 22-250 and he tried shooting some 45 gr bullets I had for my .22 Hornet. We figured 3,600 fps, but they weren't hitting the target at 100 yards. Moving the target back to 50 yards we got shrapnel. Turns out they were spinning at 259,200 rpm and just disintegrating once past the muzzle.

A round ball out of a 1:72" at 1,400 fps is only clocking 14,000 rpm by comparison. Dremel speeds.
 
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