Effect of barrel length on twist rate & bullet stabilization.

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Effect of barrel length on twist rate & bullet stabilization.

Post by KDS991213 » Sun Jan 03, 2021 5:16 pm

Can someone clarify this for me.

So as far as I understand, the longer the barrel the higher the velocity of the bullet at least upto a certain point which is determined by how much powder needs to be burned off.

I have heard people say that the twist rate of the rifling is the sole factor determine how fast a bullet will spin. According to them x amount of twist rate will cause x bullet to spin with x speed regardless of the barrel length. It doesn't matter to them if a barrel is 20 inches long & another barrel in 4 inches long as both will produce same amount of bullet spin.

If true then if we take 2 20 inch long barrel, one with 4 inches of rifling on the muzzle end & another with full 20 inches of rifling. Will the produce the same bullet spin & stabilization.

By the way i'm fully aware that it's never this simple & a lot of other factors come into play in real life.

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Re: Effect of barrel length on twist rate & bullet stabilization.

Post by eljefe » Sun Jan 03, 2021 6:50 pm

Its not just the velocity.
When a bullet exits the case mouth and after a few mm or even a few thou mm of free bore, it enters and engages the rifling, is where the spin begins. Can reach upto a a couple of hundred thousand rpm. This adds to the gyroscopic stability. Hence, rifled firearms being more accurate than the smoothbore muskets.
The Bore/Groove difference used to nominally be 8 thou of an inch. Eg, the venerable .303 had a BORE dia of .303” but the GROOVE (bullet) dia was .311”. This helped in the obturation of the bore, where bullet had a tight fit, got engraved by the rifling, none of the propelling gases escaped ahead of the bullet while it traverses the barrel. Ergo, the velocity and stability. The JUMP of the bullet, from case mouth into the freebore, until it engaged the rifling is the Holy Grail of accuracy. Some rifles are made so that the projectile JAMS into the rifling, other chambers are cut to allow a JUMP.
The debate will go on until the cows come home, about which offers better accuracy.

The ‘end of the barrel’ rifling was found to be more suited to large heavy soft lead slugs in shotguns- of bore diameter,which were known as ‘ Paradox’ and ‘Explora’. This shallow rifling allowed a bit more accuracy with a solid slug and hopefully did not ruin the birdshot pattern too much.

If this paradox type rifling was done for a rifle, the projectile would exit case mouth, no obturation to give it gyroscopic stability, gases would escape past the bullet, causing yaw and the bullet would tumble down the barrel with no guarantee that it would engage the rifling squarely at the last 4 “ in your hypothetical barrel.
It is not just the spin in RPM which is the end point. Heavy projectile will need a slower rate of twist to stabilise. E.g, a 1:14” .223 will not stabilise the newer 70 and 80 gr bullets which are in vogue with the target crowd, but will need a 1:7” or 8 to stabilise and shoot to point of aim.

After the optimal barrel length, further travel will add to the friction forces of the rifling on the bullet and may bleed off the velocity., rpm and thereby, accuracy.
So it seems there is a fine balance to achieve all these indices.
Non magnum rifle barrels are usually standardised at 22-24” while magnum calibers are nominally 26”.Some big bore Target rifles go upto 32”

BP era rifles had longer barrels because of the slow burn velocity of black powder( each manufacturer had their own formula, no standardisation of any sort) which took far longer to reach some useable velocity and stabilisation.
Importantly, modern smokeless loads are standardised internationally, and are able to achieve optimal velocity in a far shorter time. A.22LR (std velocity) is known to achieve its max velocity in about 14” of barrel travel.
Cheers and if this clear as mud, hve a read of choke rifling ;)
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Re: Effect of barrel length on twist rate & bullet stabilization.

Post by timmy » Mon Jan 04, 2021 5:25 am

Freebore, the section of the barrel between the chamber and where the rifling begins, can allow the speed of the projectile to increase before the bullet is engraved by the rifling, thus increasing muzzle velocity. Weatherby commonly had long freebore in their barrels to increase muzzle velocity.

Paradox rifling is an extreme example of this, where the rifling is only at the very end of the barrel.

This is all well and good, but there are some disadvantages to doing this:

1. As el jefe noted, obturation, or the expansion of the bullet to seal the gasses behind it, is an issue

2. Usually, best accuracy is achieved by the bullet resting against the beginning of the rifling. This is because the bullet must enter the rifled part of the barrel exactly on the rifled bore's axis, and not crooked at all. If the bullet is launched out of the barrel crooked, inaccuracy results.

3. The bullet material must be strong enough to resist "tearing" of the bullet material and having the bullet "jump" the rifling lands. This often results in copper and/or lead deposits in the barrel, clogging the grooves of the rifling. Jumping the rifling lands and barrel deposits both contribute to inaccuracy. Loading the bullet right up against the rifling can increase chamber pressure and lower muzzle velocity, but a long throat (or, at the extreme, a paradox bore) will allow higher velocity to develop. If velocity is increased beyond the bullet's ability to engrave and grip the rifling, accuracy can be affected.

4. The twist of the rifling controls how fast the bullet spins for a given velocity. It is usually best to spin a bullet fast enough to stabilize it, but no more. Too fast of a twist, or spin of the bullet, can result in "over-stabilization. You will note that the bullet's construction, similar to (3.) above, must resist the tendency of the bullet to fly apart due to thousands of rpms. Also, any imbalance of the bullet from errors in the manufacturing process, or from damage in handling or cycling through the action, will cause an out-of-balance condition as the bullet is rotated, which, in turn, will cause the bullet to wobble in flight. The faster the bullet is rotated, the more the imbalance will affect it, by geometrical factors.

5. How fast must the bullet be spun to stabilize it? Diameter plays a role here, as does velocity, and also the length of the bullet. a long bullet requires a faster twist to stabilize than a shorter bullet. This is also somewhat related to weight: For instance, the 1:10 twist of the 30-06 is said to stabilize bullets up to 220 grains, but heavier bullets can be unstable. Similarly, some folks will order a barrel in 1:12 twist, if they shoot only lighter bullets of, say, 150 grains from their 30-06. Or, a 308 or 300 Savage rifle might have a 1:12 twist, because lighter bullets are often used with these cartridges. However, the major issue here is not the bullet weight, it is the length. A round ball, often used in muzzle loaders, is not a "long" bullet at all, and round ball muzzle loaders which usually have large bores, often have a 1:60 or 1:72 twist. (Some modern muzzle loaders will have a 1:48 twist, or faster, so the gun can be used with conical, minie, or other type of bullet. However, when shooting a round ball, the slower twists are more accurate.)

If you wish to get into the mathematical weeds of this issue, here's a Wiki to read: https://en.wikipedia.org/wiki/Miller_twist_rule

If you are lazy, like me, you can google this subject and find any number of calculators for finding an optimal twist rate, and sometimes the math behind them. Here's a few:



https://www.vcalc.com/wiki/AndrewBudd/G ... Twist+Rate

I studied this stuff deeply when considering a bullet mould for my Husqvarna 45-70 rifle. As it is a light rifle (VERY light!), a heavy bullet has penalties, but perhaps may be more accurate. In the end, I just gave up on it and figured I'd stick with my "collar button" bullets, intended for gallery, or .22 style shooting, and use my Martini Henry if I wanted to hog hunt with a big boomer.
“The principle of self defense, even involving weapons and bloodshed, has never been condemned, even by Gandhi.” - Martin Luther King, Jr.

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