Twist rate vs barrel length

[Shadow9mm]

Was talking to a friend, had a theory that I'm researching. Basically you want the bullet to make 2 full rotations or more in the barrel. For 223, with a 16in barrel, minus 2 for the chamber, a 1:7 twist gives you 2 rotations. For a 1:8 twist you need a 18in barrel. For your standard hunting rifle generally you have a 1:10 twist with a 22in barrel, giving you 20in of rifling and 2 full rotations.

I know twist rate is related to bullet length/weight. But does barrel length / twist rate effect accuracy?

Any legitimately or fudlore?

[Pumpkin]

A longer barrel in theory SHOULD give you more velocity thus increasing bullet rpm which SHOULD help stabilize long for caliber bullets.
Not too sure about that 2 full bullet rotation theory though.

[JohnKSa]

A longer barrel in most centerfire rifle calibers will mean higher muzzle velocity.

Higher muzzle velocity for a given twist rate will mean a faster spin.

Faster spin means you can stabilize longer bullets in that bore.

So, a longer barrel would, in most centerfire rifle calibers result in more stable bullets. To a point. At some point you can spin the bullets too fast--although different sources differ as to exactly what that means. So really, there's a range of spin that you want, it's not strictly a matter of faster spin is better. Berger bullets used to (may still) have an explanation on their website of why they typically recommend that barrel twist rate be limited to what is sufficient to stabilize a bullet vs. just going as fast on the twist as you can in hope that faster is always better.

I can't think of any reason why the bullet would need to make a certain number of rotations in the bore. As far as I know, the only thing that matters for stabilization is the spin rate as the bullet exits the muzzle.

[mikejonestkd]

A quick note to muddy the waters even more - a longer barrel will gain velocity and need less twist than a shorter one, and a more powerful cartridge can get by with less twist than a lesser cartridge for the same bullet

a good comparison is the .223 rem and the .22-250

A .22-250 typically has a 1 in 14" or a 1 in 12" twist and a normal barrel length is 26"
I can stabilize most bullets up to about 62 GR because they are being pushed at over 3500 FPS.

a 1 in 12" twist on a .223 remington 16" barrel would not be able to stabilize a 62 GR bullet at roughly 2900 FPS.

[Jim Watson]

There is no "rotation" requirement. If there were, how would you ever hit anything with a pistol?

In my opinion, working on the ragged edge where velocity matters is not rewarding. I did it one time, though. My .22-250 14 twist will shoot a 60 gr JSP pretty well. But a 60 gr JHP is wild. It is no heavier, but is longer, which is what counts.

[tangolima]

The number of revolutions of the bullet in the bore has no bearing on its stability in flight. It doesn't even necessarily leads to longer barrel for higher mv.

Higher MV does increase the bullet's rpm, but the force trying to tumble it also increases. They sort of cancel out each other, almost. The end result is that stability increases rather slowly with speed.

In the old Greenhill's equation, MV is not a factor. Rather the limit changes from 150 to 180 when MV reaches 2800fps. The more modern Miller's equation include the 3rd root of MV, also reference 2800fps. Any 3% increase in MV (100fps ish faster) will increase the stability factor by 1%.

It is a lore people choose to believe I think. It seems credible as it matches what we see.

-TL

Sent from my SM-N960U using Tapatalk

[Shadow9mm]

Originally Posted by Jim Watson

There is no "rotation" requirement. If there were, how would you ever hit anything with a pistol?

In my opinion, working on the ragged edge where velocity matters is not rewarding. I did it one time, though. My .22-250 14 twist will shoot a 60 gr JSP pretty well. But a 60 gr JHP is wild. It is no heavier, but is longer, which is what counts.

But pistols are noticeable less accurate than rifles. Not super familiar with competition pistol shooting but don't they measure groups in inches at 25y? Never heard of a pistol groups being measured in moa.

[Pumpkin]

Thompson Contender

[taylorce1]

Originally Posted by mikejonestkd

a 1 in 12" twist on a .223 remington 16" barrel would not be able to stabilize a 62 GR bullet at roughly 2900 FPS.

This is not entirely true, because not all bullets of a certain weight require the same twist rate. Bullet length is what requires a change in twist to stabilize them. I've had 1:12 twist .223 Remington shoot 64 gr flat base bullets well enough that I'd not hesitate to use them. The 63 grain Sierra SMP is a great bullet for stabilizing in slow twist .224 caliber rifles, along with the 64 grain Nosler Bonded Performance. Then you have bullets like the 55 grain Barnes TSX that will often keyhole at 100 yards out of 1:12 twist .223 Rem rifles.

[tangolima]

Examining the Miller's equation, every 1% increase in bullet length requires 9% increase in MV to maintain the same stability factor. It is a rather difficult battle to fight.

-TL

Sent from my SM-N960U using Tapatalk

[mikejonestkd]

his is not entirely true, because not all bullets of a certain weight require the same twist rate

my response was just a rough guideline, not dogma.
YMMV

[JohnKSa]

But pistols are noticeable less accurate than rifles.

This is due to a number of factors that have nothing to do with rifling.

1. Shorter sight radius which means that small amounts of sight misalignment mean larger error on target.
2. Coarser sights and lower magnification optics which means that the aiming point tends to be less precise than with rifle sights.
3. More play between parts. For example, in a typical recoil-operated pistol, the sights are on the slide which is not fixed to the barrel.
4. Typically handheld vs. shot from a rest or from the shoulder which means more movement of the gun during the sighting/firing.

In "handguns" where these variables are eliminated, (Thompson Contender, Remington XP100, etc.) accuracy is similar to rifles. In fact, there's evidence that given enough twist/muzzle velocity to provide stability, shorter barrels are somewhat more accurate because for a given barrel diameter they have less "whip" than a longer barrel.

[Paul B.]

"I know twist rate is related to bullet length/weight. But does barrel length / twist rate effect accuracy?"

The twist rate will depending on the length of the bullet. Barrel length will affect velocity to some degree.

How ever, requiring two complete rotations to properly spin the bullet??? I think not. Take the S&W M60 with a 1 7/8" barrel and place in on a Ransome rest and shoot strictly for accuracy by eliminating most of if not all human error. You'll find that short barreled revolver can be a lot more accurate than you might believe and there is no way for the bullet to take two full turns in that barrel. I'm not sure it can do even one full turn, if truth be told yet the accuracy is there. I'm thinking maybe 50 to 100 yards from that rest. Do that same test with a S&W K38 with 8 3/8" barrel and I doubt the accuracy will be much different although velocity will have increased.
Paul B.

[Jim Watson]

In fact, there's evidence that given enough twist/muzzle velocity to provide stability, shorter barrels are somewhat more accurate because for a given barrel diameter they have less "whip" than a longer barrel.

Well, not THAT short. The Houston Warehouse Experiments settled down to 21.75" for typical benchrest rifle calibers. But there has been some very fine shooting done with Contenders and XP-100s. There was the guy who made his Contender grips out of unfinished wood because the bottom of his rest was covered with sandpaper and he rubbed the gun butt into a perfect fit.

[JohnKSa]

The Houston Warehouse Experiments settled down to 21.75" for typical benchrest rifle calibers.

It's been a long time--was the barrel length exclusively about accuracy or was it a combined accuracy and muzzle velocity consideration?

[Shadow9mm]

Originally Posted by JohnKSa

It's been a long time--was the barrel length exclusively about accuracy or was it a combined accuracy and muzzle velocity consideration?

my guess is there is also a stiffness vs weight consideration. you make a barrel longer, to maintain the same stiffness the barrels diameter has to increase. I know some competitions guns have weight limits, but also there is just practical weight limits to the gun being too big and heavy to be practical.

I remember a shot show video from a few years back at the barrett firearms. They had some special version of the semi auto 50cal for the military, had a shorter barrel. Said it was actually more accurate because it was stiffer due to being shorter.

I think the TC Contenders answer the question fairly well, makes sense. Was an interesting question, and seemed to line up at a quick glance, but after a little looking I think I'm chalking it up to fudlore.

[taylorce1]

Originally Posted by Shadow9mm

But pistols are noticeable less accurate than rifles.

There are a lot of reasons pistols are less accurate than rifles. Mainly it's because it doesn't need the same precision as a rifle, as a pistols have mainly been designed for close range work. So the amount of precision needed to hit a basketball sized target at 25 yds +/- , is far different from hitting a basketball at 100-800 yds.

They're lighter and more manuverable, making them harder to steady when compared to a rifle. The sighting systems usually aren't as refined. They have a shorter sight radius when using iron sights making them less precise.


Then the ammunition hasn't been as developed as rifle. The need for high BC bullets and powders and loads that produce more uniform velocites isn't as important in a pistol again because of the range they're typically used at. They're exceptions, as some of the competition/specialty postol shooters with their highly customized and tuned pistols are surely capable of printing sub MOA groups with firearm of choice.

Here is a LINK to help you better understand how accurate some pistols can be in the hands of a world class competitors. I learned some of this several years ago when my daughter was learning to compete in 10m air pistol. I wish she would have continued, but she lost interest before she was able to compete without using a rest. It didn't help that when we moved for my job she would have had to spend more than four hours commuting to and from the Olympic Training Center two nights a week. It was just difficult on an 11 year old.

[101combatvet]

From memory, every inch of the barrel gives you about fifty feet per second of velocity.

[s4s4u]

From memory, every inch of the barrel gives you about fifty feet per second of velocity.

Actually no, not even close. In fact, it isn't lineal at all. As the barrel gets shorter, the amount of velocity loss increases per barrel inch. Going from a 26 to a 22 inch barrel might lose a total of 100 fps. Going from a 16" barrel to a 12" barrel will result in 2 - 3 times that loss, depending on the ammo.

As to the OP, no, there is no requirement of 2 revolutions in the barrel to shoot accurately. Match grade 22LR barrels might get 1 full revolution. Most match grade pistols won't even make 1/2 of a revolution. RPM is what matters, regardless of how you get there.