Twist Rates

[jaybez101099]

After reading more than my share of posts on here the twist rate factor still has me puzzled. Is the 1/30 one rotation every 30"s?
And if so does that mean a 24" barrel the sabot only is performing 3/4 rotation? If you are planning on shooting only a saboted bullet is a slow or fast rate better? I noticed the whites are faster than most with a 1/24. Does a twist rate have a matching load preference as in 1/26=120g. I googled it and never found a direct answer other than PRB's were mostly 1/60ish but that didn't help me.

[halfbakedi420]

i think you are correct with it being 1 full twist every 30 inches...and yes 3/4 +/- in a short barrel, couldn't remember how long you said but it wasn't 30 inches. i dont think so on the bullet weight though ig 1/26 and 120 gr bullets....think of em like flying bricks...the bp aint puttin out 3k fps ...more like throwing a football

[Big Z]

Greenhill formula might be something interesting for you to read about..

[lemoyne]

Originally Posted by jaybez101099

After reading more than my share of posts on here the twist rate factor still has me puzzled. Is the 1/30 one rotation every 30"s?
And if so does that mean a 24" barrel the sabot only is performing 3/4 rotation? If you are planning on shooting only a sabot-ed bullet is a slow or fast rate better? I noticed the whites are faster than most with a 1/24. Does a twist rate have a matching load preference as in 1/26=120g. I googled it and never found a direct answer other than PRB's were mostly 1/60ish but that didn't help me.

The length of the barrel really dose not have a lot to do with the twist. The length of the bullet and the velocity have to match up with in reasonable limits so the the bullet is stabilized like a spinning top. That is what keeps it form going end over end. That is why a smooth bore has trouble getting any accuracy at a distance.
Greenhil formula
Twist = 150 X D2/L
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
This applies to muzzle loaders and if you were to us it for a cartridge gun that had a velocity of over 2800 FPS then the constant would be 180.
Think of it this way if your muzzle loader had a standard twist of 1 in 28 that is one complete spin every 28 inches if it had a velocity of 2000 FPS the the bullet is spinning 4286 times a minute. If you check with the green hill formula this will stabilize a bullet from about .75 to about .95 in a 45 caliber.
Hope this helps. Lee

[sabotloader]

jaybez101099


Here is some good evening reading

Twist Rate
Bullet stability depends primarily on gyroscopic forces, the spin around the longitudinal axis of the bullet imparted by the twist of the rifling. Once the spinning bullet is pointed in the direction the shooter wants, it tends to travel in a straight line until it is influenced by outside forces such as gravity, wind and impact with the target.
Rifling is the spiral or helix grooves inside the barrel of a rifle or handgun. These grooves were invented a long time ago, perhaps as early as the 14th century. However, the smooth bore, using the round ball, was the choice of weapons for warfare even through the American Revolutionary war. The smooth bore musket could be loaded faster than the rifle and didn’t foul, as bad, with the combustion products of black powder.
The rifling grooves helix is expressed in a twist rate or number of complete revolutions the grooves make in one inch of barrel length. A 1in10 or 1:10 would be one complete turn in 10 inches of barrel length.
How important is twist rate? David Tubb, a winner of several NRA High Power Rifle Championships, was using a .243 rifle with a 1 in 8.5 twist barrel. He wasn’t able to get consistent accuracy until he changed to a rifle barrel with a 1 in 8 twist. The ½" twist change made all the difference between winning or losing the match.
A term we often hear is "over-stabilization" of the bullet. This doesn’t happen. Either a bullet is stable or it isn’t. Too little twist will not stabilize the bullet, while too much twist, with a couple of exceptions, does little harm. Faster than optimum twists tend to exaggerate errors in bullet concentricity and may cause wobble. The faster twist also causes the bullet to spin at higher rpm, which can cause bullet blowup or disintegration because of the high centrifugal forces generated. For example, the .220 Swift, at 4,000 fps., spins the 50-grain bullet at 240,000 rpm.
One of the first persons to try to develop a formula for calculating the correct rate of twist for firearms, was George Greenhill, a mathematics lecturer at EmanuelCollege in Cambridge, England. His formula is based on the rule that the twist required in calibers equals 150 divided by the length of the bullet in calibers. This can be simplified to:
Twist = 150 X D2/L
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
This formula had limitations, but worked well up to and ‘in the vicinity of about 1,800 f.p.s.’ For higher velocities most ballistic experts suggest substituting 180 for 150 in the formula. The twist formulas used in the Load from a Disk program, featured at this web site, uses a modified Greenhill formula in which the "150" constant is replaced by a series of equations that allow corrections for muzzle velocity from 1,100 to 4,000 fps.
The Greenhill formula is simple and easy to apply and gives a useful approximation to the desired twist. The Greenhill formula was based on a bullet with a specific gravity of 10.9, which is about right for the jacketed lead core bullet. Notice that bullet weight does not directly enter into the equation. For a given caliber, the heavier the bullet the longer the bullet will be. So bullet weight affects bullet length and bullet length is used in the formula.
To measure the twist of a barrel, use a cleaning rod and a tight patch. Start the patch down the barrel and mark the rod at the muzzle. Push in the rod slowly until it has made one revolution, and then make a second mark on the rod at the muzzle. The distance between marks is the twist of your barrel.
To see how this works out, assume you bought a .222 Remington rifle and you measured the twist rate as described above. The twist was 1 in 14. You have two .224 bullets you want to use, the 70-grain Speer SPS and the 50-grain Hornady SX. The Speer bullet measures .812 inches in length and the Hornady measures .520 inches. Using the formula above we calculate the following twist rate:
Speer 70-grain: 1 in 9
Hornady 50-grain: 1 in 14
These calculations show that the 50-grain bullet will be stabilized, but the 70-grain won’t. Sure enough, when you try these bullets out, the 50-grain shoots ¾ MOA while the 70-grain won’t group on the paper at 50 yards. Twist is important!