Barrel twist rate effect on speed?

[Deja vu]

Lets say I have 2 identical barrels (I know this is not possible). But one barrel is a slow twist (say 1 in 20) and the other is fast twists (say 1 in 10). Which barrel should shoot faster?

What effect on accuracy will a twist rate too fast have? Such as a bulet that is ideal at 1-15" going through a 1-10" barrel?

Just trying to come up with some ideas.

 

[CarJunkieLS1]

My feelings would be that given everything the the exact same, which we all know isn't possible, is that the slower twist will have a higher velocity because it will take less friction to push that bullet down the barrel.

This is my theory and I'm no expert so its my $.02. I'm sure there is some one that has the answer with the science to know the answer.

 

[PewPewPachew]

The barrel with a slower twist rate should be capable of more velocity, as it has less resistance for the bullet as it travels down the bore. The opposite is true for a faster twist rate.

*Edit* It may be pertinent to point out "progressive" or "gain" twist barrels. These have a slower twist at the throat, and gradually get faster towards the muzzle. The theory is that you will have lower pressure and resistance in the beginning, potentially increasing velocity, and decreasing throat erosion. By the time the bullet exits the muzzle, it should have been spun up to the proper speed by the rifling at that end.

I have never personally had an issue with shooting a bullet that was too heavy for the twist I was using, as I never really pushed too far beyond recommended weights. Some barrel and load combos also seem to work, that shouldn't have.

However, I have loaded bullets designed for a lower twist rate and/or velocity for rifles with higher twist rates. Particularly the .224 flavor. I had a 22-250, with the older, slower 1:14" twist. It loved 40 and 50 gr pills.

Friend had grabbed a bolt gun in 223, had a 1:12" twist. We loaded up a variety of weights and powders for it. It did really well the 55-65 gr pills. The 50 gr ones started getting more spread to them. The 40 gr bullets were all over the place. I don't think you could call it a grouping at 100 yds, and it looked like some were wobbling or about to tumble in flight.


So too much twist can be as bad as not enough.
*Edit Again* Although there are recommendations or lots of equations and stuff to go by, I still stand by what I mentioned earlier. Some bullets at whatever given velocity, can function well out of a barrel with twist that doesn't match. It shouldn't work, but sometimes it does.

 

[TimW77]

"The barrel with a slower twist rate should be capable of more velocity..."

Exactly...

General physics and aerodynamics...

There is "X" units of total energy in the cartridge...

The faster the twist, the MORE of that energy is needed to spin the bullet leaving less total energy and resulting in LESS VELOCITY...

T.

 

[FrankenMauser]

Slower twist rates result in higher velocities, all else being equal.

How much that matters, is debatable, however. The much-maligned and contrastingly beloved .444 Marlin, for example, is typically found with a 1:38" twist rate for pre-1998 Marlin and Winchester rifles and a 1:20" twist rate for post-1997 Marlin rifles (and H&R and Remlin/Marlington rifles). But the difference between twist rates, for a given barrel length, is typically less than 20 fps -- far less than one would expect.

How, exactly, you can calculate the difference between twist rates for a given cartridge, I have no idea. I never finished my engineering courses.


I can tell you from various experiences that too fast of a twist rate is just as bad as (if not worse than) too slow of a twist rate.
Yet again with the .444 Marlin example: If you launch a 400+ gr bullet into a 1:20" twist 444 barrel, it will 'skid' across the rifling for a couple inches, before 'biting' and rotating with the rifling as designed (proven across multiple bore and groove diameters with bullets up to 0.004" over groove diameter).

And, of course, I always like falling back to .220 Swift. Push bullets too fast (too much rotational velocity from the combination of velocity and twist rate), and you get bullets that self-destruct mid-air.
Shooting a max-velocity 40 gr HP out of .220 Swift in -15 F weather can result in a bullet that never makes it to the target. ... And that's in a 'standard' twist rate for the cartridge!

 

[jmr40]

It seems logical that the slower twist should be faster. But modern smokeless powder has a tendency to increase the speed of a projectile when pressure or resistance is increased. My 308's shoot 155 gr bullets about 30-40 fps faster than 150 gr bullets with the same powder charge. It is well known that certain brands of bullets have more bearing surface on the sides of the barrels than others. Those bullets increase pressure and need lower powder charges to reach the same speeds. With the same powder charges they may be over a max charge compared to another bullet.

While I don't have any hard data to say for sure either way I'm going to go with it makes no difference, or very little difference. And just maybe the faster twist will result in slightly more muzzle velocity.

I have also read that a faster twist barrel will wear out after fewer rounds. Once again that seems logical, but I've seen no hard data to confirm this either.

 

[ShootistPRS]

for target shooting you want a bullet that is close to minimally stable. That will keep the nose of the bullet pointed in the direction of the bullet path.That tends to keep the velocity more and it makes for a nice clean hole to measure.

In hunting bullets you want more spin to aid in the expansion of the bullet. There are extremes on both ends of the scale but when in doubt use a faster twist rate.

You can't "over stabilize" a bullet but you can spin it so fast that combined with the aerodynamic heating it will disintegrate before it reaches the target.

 

[SSA]

If this link works, here's some info.

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

 

[Unclenick]

ShootistPRS,

Yes with caveats. Truly minimum stability is a gyroscopic stability factor of 1.0. So if you have that, the bullet won't tumble, but the bullet will be way past the target before it slows enough to increase stability factor enough to stop coning and settle into the yaw of repose. So drag will be higher until then and group size is generally pretty poor with such bullets as they easily pick up radial drift deflection as they go nutating through small atmospheric irregularities. Sierra recommends a minimum gyroscopic stability factor of 1.3 for "hunting accuracy", and 1.4 for target accuracy. They recommend a maximum of 1.7 for best match accuracy and 3.0 for hunting, though their match bullets are good enough that I've seen them do very well with numbers over 2.0.

It's also correct that there is no such thing as being overstabilized, but, as you pointed out with the disintegrating bullet example, there is such a thing as over-spun, and there are actually two more consequences of that:

One is core stripping. Harold Vaughn put small magnets in bullet noses (.270 Winchester) and shot them over a magnetometer coil and found that above a certain level of acceleration the jacket of his cup-and-core bullets would start slipping over the lead core and the bullet would then show lower spin rate on the instrument than a lower velocity produced. So he'd found a limit to how much angular acceleration that conventional bullet construction could stand in his caliber.

The other consequence is that if the bullet isn't perfectly symmetrical in its mass distribution and perfectly straight in the bore, groups will open up because of the off-trajectory drift the bullet then picks up when it exits the muzzle. This is just the angular velocity of the mass center around the bore axis times its radius off the center. The bullet drifts at that velocity tangent to whatever point on the muzzle it was closest to at exit and in the direction of spin.

It is to minimize the above effect that shorter range benchrest shooter like the relatively short radius flat base custom bullet, as they require the lowest rifling pitch to reach a stability factor of 1.4 (or some authorities prefer 1.5).


To the OP's question:

First the velocity. Frankenmauser is correct, in principle if all else is exactly equal, faster twist will shoot faster for the same reason that shooting a moly-coated bullets (less friction) typically costs you 20-50 fps for a given powder charge. This is due to lower pressure needed to swage the bullet into the rifling and to the fact the progressive pressure build-up has to chase the bullet down the barrel, and the reduced friction means the bullet scoots ahead a little, increasing expansion so the gas has to raise pressure in a larger space.

For the above reason, even though the faster twist causes the bullet to take more energy out of the system by greater angular (rotational) acceleration, the added friction building pressure faster overwhelms that difference. In another thread I recently calculated the energy of rotation for a 50 grain .224 bullet. I modeled it in my CAD software which automatically finds moments of inertia (I was way too lazy to work it out mathematically). Based on that, the bullet at 3200 fps acquired about 1.25 ft-lbs of angular (rotational) kinetic energy from a 14 inch twist and about 5 ft-lbs from a 7 inch twist. Double the rate of rotation and you quadruple the energy of rotation as it is proportional to the square of angular velocity, analogous to muzzle kinetic energy being proportional to the square of translational (displacement or distance covering) velocity. In any event, the ME was about 1137 ft-lb, and if you could subtract the rotational energy from the muzzle energy it was about a 1.75 fps difference for the 14" twist and about 7 fps for the 7" twist. Small percentages of the muzzle velocity. But you can't subtract it because of the faster pressure rise effect, as I said, overwhelming with added total energy.

 

[JeepHammer]

That's the scientific version, and I agree.

The PRACTICAL side of it is...
1. Rifling cuts into the jacket, super fast twist rates can & will strip thin jackets off bullets, but the jacket comes off in strips, not evenly.

I found this out by missing the backboard of targets altogether shooting light weight/thin jacket varmint bullets through a fast twist barrel at high velocity,
Then finding strips of jacket on the ground.

When you have an 'Air Brake' sticking out from one side of the bullet the bullet don't go where you want it to.
When you have a copper strip missing off one side of the bullet making the bullet lopp-sided while spinning 200,000 RPM it don't go where you want it...

2. Feet Per second velocity times twist rate of barrel.
Most of the 'Inherently Accurate' modern bullets spin in the general neighborhood of 150,000 to 200,000 RPM.
Somewhere between 250,000 & 300,000 RPM all but the thickest jackets shred.

You CAN spin a bullet too fast as well as too slow.

3. For any given 'Caliber' (bore/rifling) diameter, to increase bullet weight you MUST increase length.
For instance, the 0.218" bore & 0.223" rifling grooves aren't going to get any bigger to allow for more weight,
So the bullet simply has to get longer to add weight.

Adding Length adds load bearing surface to the sides of the bullet, load bearing surface that grips rifling grooves.

The heavier/longer the load bearing surface on the bullet is, the better it will grip faster twist rate barrels.
Barrels that shoot 0.223" diameter bullets at 65 grains well will probably try to strip the jacket off a 35 grain bullet in the same diameter & same velocity,
And a lead (non-jacketed) bullet has no chance of holding rifling.
The rifling will simply strip the outside of the lead bullet, sizing it to 0.218" (bore) size at the same velocities/pressures as a jacketed bullet.

This is the reason a large percentage of loaders find the rifle shoots more accurate with a reduced from maximum powder charge.
Velocity has a lot to do with how that bullet initially sizes to the bore/rifling, and how well that bullet holds the rifling (Internal Ballistics),
Along with how that bullet acts once it exits the muzzle, adjusting velocity for the bullet/barrel is what reloading for accuracy is all about...

Adjusting powder/pressure for best internal/external performance for your given powder choice, bullet choice & barrel design...

 

[JeepHammer]

When someone talks about 'Inherent Accuracy' I immedately think of .22LR/.22 WMR or the great granddaddy of the 'Super' .22 center fires, the .218 Bee.

Both are 'Long' compaired to diameter of the bore,
Both are light weight bullets compared with modern offerings,
Both stabilize (sleep) at their respective nominal velocities,
Both have fairly 'Slow' twist rate barrels compaired with modern offerings.

35 to 45 grain bullets in a .218 Bee is laser accurate out of 1:14" twist barrel,
While 1:14" twist barrel is unsuitable for heavier bullets.
Average muzzle velocity of the .218 bee is slightly slower than .223 Rem,
And the original .223 Rem had a 1:14" twist rate, but was intended for 'Varmint' weight bullets.

As people used heavier bullets, the twist rate had to increase to stabilize those bullets, which has got completely out of hand in my opinion...

When I'm building a .223 Rem. chambered varmint rifle (pin point accurate out to about 400 yards) I'm usually looking for a barrel that matches the lighter weight varmint bullets, say no more than 1:10" twist rate.

The light weight bullet will loose velocity to wind resistance fairly quickly, so I say 400 yards, and around here you normally can't see varmints more than 400 yards (rolling hills, brush), So the round & rifle match the application.

When someone wants to shoot 600 yard targets, heavier bullets, faster twist rate, relatively the same muzzle velocity.

As Unclenick said, getting, and maintaining pressure until the bullet exits the barrel is a big deal (internal ballistics), so the 600 yard shooter isn't nessairaly going to shoot the same powder a slower twist rate/lighter bullet shooter is going to use to achieve the same muzzle velocity.

That's why we do 'Ladder' tests to find out what small changes in powder best works with the bullet/barrel we are using.

All things being EXACTLY equal, with the exception of twist rate, the slower twist barrel will show more muzzle velocity,
BUT,
That is a therotical question, and added velocity certainly isn't a reason/consideration for choosing a slower barrel twist rate.
WAY too many other factors to consider into the mix to focus on velocity alone.

 

[Unclenick]

All things being EXACTLY equal, with the exception of twist rate, the slower twist barrel will show more muzzle velocity,

Other way around. Just as adding lubrication to bullets or bore surfaces to reduce friction will lower velocity, so will lower angular acceleration due to slower twist. You have to move to a faster burning powder or to a larger quantity of the existing powder to keep up with the more easily developed rate of expansion the lubrication provides if you want velocity to match.

 

[bfoosh006]

Wow... the linked "AccurateShooter" article says it all for me.

I have higher SD numbers higher then the "Twist Rate" numbers.... 11 FPS is nothing.

 

[Unclenick]

Yes, but I'll caution you that it's backward from Frankenmauser's results and some I've seen. Litz went the extra mile to get the barrels all from the same maker, but they still won't be microscopically identical and most of them are 10" twist and the variation among those (6 fps) is smaller than the standard deviations and probably smaller than the standard error in some instances. The physics he talks about is what I described in figuring out rotational energy for the .223 Rem rounds.

So, it's very difficult to tell the limitations of what he measured and it is why he cautiously added that uncertainty is always an element of live fire results. I'd like to see him repeat the testing with multiple powder burn rates to get a wider sense of the slowing vs. speeding up effects. The slow powders should be more vulnerable to being slowed by slower twist rate. I also wish he'd mentioned his sample size.

 

[McShooty]

I believe jmr40 has a point. Resistance to bullet travel in the barrel while powder is burning may raise pressure and actually increase the velocity of the bullet. My anecdotal evidence is that my experience with Marlin 336 30-30s is that, with their 1-10 twist and micro-groove rifling, they give somewhat higher than expected velocity in a 20-in bbl. Most 30-30s have 1-12 twist. Less anecdotal, a test of seven factory, 150-gr loads in a 20-inch 336 gave an averagev velocity of 2335 fps (advertised 2390 fps for 24-in bbl). The same loads in a 24-in Winchester 94 (1-12 twist) gave an average of 2351 fps. One would expect a 4-in difference in barrel length would give at least a 100 fps difference in velocity.

 

[ShootistPRS]

I think you would find that the internal dimensions of the barrel will affect muzzle velocities more than a change in twist rate.

I had two identical revolvers (357) that I fired the same ammunition through and one had velocities that were 150 - 200 fps faster than the other. These were made by the same manufacturer, had the same twist, and the same length. I would think that a few ten-thousandths difference in bore diameter is what made the difference in velocity. I never tried to slug the bores to find out as it was not important to me why - it was enough to know that there was a difference.