Gain twist rifling

My personal opinion is that the change in the angle of the rifling engraving as the missile goes up the barrel distorts the bullet too much for tip-top top accuracy.

After all, you're already squooshing the bullet with regular rifling, but with gain twist rifling, you're squooshing it even more. Squoosh here and it comes out there. ("Squooshing" is a technical term used only by expert ballisticians in perfesshinel journals.)

With "straight" rifling the bullet has to accelerate its angular velocity (RPM) gradually anyway as it starts from zero or near zero velocity (and RPM) to the muzzle at max velocity and therefore max RPM. I note that it is lot easier to make gain twist barrels now that they can be hammer forged instead of being cut or button-rifled, so there may be a psychological marketing advantage there. Think: "New, Improved Distilled Water."

^Personal opinion only, not subjected to actual testing.

The only situation where the missile would not be distorted as the rifling angle changes is with a round ball or (as in the case of artillery) there are thin driving bands on the projectile. Any advantages to gain twist might show up better there.

^Caution : Opinion unsupported by test.

There have been individual weapon cartridges designed for gain twist or tapered bore barrels with flanges, but again, my basic operating principle is keep bullets simple. (See "Cartridges for Collectors, Vol 2," by Fred Datig, page 60, the 14/9mm Gerlich cartridge as an example.)

Terry, 230RN

Not sure but when I was an ordnance mechanic on M61A1 Vulcan 20mm cannon's used on the F-15 Eagle, all the barrels were rifled with a gain twist. Pietta also makes a high end, target type New Model Army percussion revolver with gain twist rifling. It is fairly uncommon otherwise that I've seen.

It allows for a higher twist rate at higher velocities without the risk of stripping the rifling.

That is why all aircraft cannon use gain twist, 20mm, 25mm, and 30mm.

So far as potential accuracy goes, for standard rifle calibers and projectiles, there is no demonstrable advantage for gain-twist rifling. What is not generally understood or mentioned is that the effect of the gaining pitch on the bullet is a shearing one, as the angle of the rifling continually changes through the length of the barrel - additionally, the grooves actually become narrower as they advance toward the muzzle, adding more stress on the bullet, jacketed or lead alloy. If there was any real advantage in favor of gain-twist, you may be sure that the benchrest shooters would have adopted it long since: they haven't, and their level of accuracy is as good as it gets.
It is true that gain-twist barrels are used in some smaller caliber cannon, but the projectiles are all steel-bodied with a single (relatively narrow) driving band of a tough bronze alloy, which does not suffer so great an effect in travel through the bore.

PRD1 - mhb - MIke; barrel maker, retired

Yes, thanks for confirming what I said in the second post of the thread. I appreciate your remarks.

LaneP said:

Not sure but when I was an ordnance mechanic on M61A1 Vulcan 20mm cannon's used on the F-15 Eagle, all the barrels were rifled with a gain twist. Pietta also makes a high end, target type New Model Army percussion revolver with gain twist rifling. It is fairly uncommon otherwise that I've seen.

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Yes, the cap and ball percussion revolver would fall in with my remarks about round balls not suffering from the angle changes going through the barrel.

Terry, 230RN

Worked great on black powder muzzle loaders with patched round balls.

The two types of gain twist rifles of which I have some information are muzzleloaders and the 6.5x52mm Carcano.
Muzzle loaders typically use a pure lead bullet (ball or conical); so soft they would "strip" (slide through) the first length of rifling. Or they were perceived to done so. I really don't doubt the old timers who felt that way, they weren't dummies.
The Carcano was made that way for the same reason or suspicion. The bullets for the round were - to my knowledge - always jacketed, but I don't know if the initial jackets properly handled the job. Again, the possibility of perception raises it's head.
I have no knowledge of the cannon so equipped.

With modern firearms, modern jacket material and small arms velocities, I don't think there's much need for gain twist in small arms.

Doesn't the S&W 460 revolver have gain twist rifling?

lysanderxiii said:

It allows for a higher twist rate at higher velocities without the risk of stripping the rifling.

That is why all aircraft cannon use gain twist, 20mm, 25mm, and 30mm.

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Yes believe this is the biggest advantage,

troy fairweather said:

Yes believe this is the biggest advantage,

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The biggest disadvantage is that with gain twist rifling you cannot use CHF or a button to make the barrel.

Bob Willman said:

Doesn't the S&W 460 revolver have gain twist rifling?

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That's what I thought.

Bartlein makes gain twist barrels for modern target rifles and I read that Krieger has, too.

Harry Pope thought they were worthwhile but as part of his complete lead bullet target shooting system.

I don't know, so I'm asking. Could the gain twist in artillery have something to do with fuze setting or arming?

Correct me if I'm misremembering. I think the early 1st gen S&W M&P15 Sport ar's had a variable rifling although I can't recall the rates. Perhaps 1:9 increasing to 1:7? vice versa? Regardless, was touted as able to stabilize a wide(r) variety of 5.56 bullet weights.

-jb, has one, but can't find documentation

I have a 6.5x55 sweede- carbine with a 18" gain twist barrel (orig barrel) sporterized rifle- that is a tack driver- with 140 gr bullets- don't know if the gain is an advantage - but it is definitely not a disadvantage.

I can confirm that gain twist can have drawbacks. I know for a fact that plated bullets are not as happy when pushed out of gain twist barrels.

Can demonstrate it with two barrels, from the same manufacture, one gain twist the one fixed. They fail in the gain twist at lower velocity’s than “regular” barrels.

I think there might be a reduction of leading with them shooting cast though. They always seem cleaner but I haven’t tested enough to offer any proof.

230RN said:

Could the gain twist in artillery have something to do with fuze setting or arming?

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More about not shearing the driving band from the shell. As long as the required RPM is reached the fusing sorts itself out (there's an element that rotates out, much like how a governor works, to keep the round safe in the barrel).

This is less of an issue in aircraft cannon that use impact fuses, where the setback is what arms the fuse. In timed fuses, one does not want the timing to start prematurely--e.g., in the tube.

The engineering theory of it is that the friction from the imputed torque is greater in "constant" rifling versus that of "gain twist." Practice shows that the additional machining effort does not achieve that great an end result. Excepting the case of high-velocity aircraft cannon where there's a different balance wanted.

The reliability wanted when spitting out 100 or 150 rounds at 6000rpm while flying >600kts makes the additional machining effort worthwhile. The three-barrel rotary helo cannon were first to this; the single-barrel "chain guns" very much cemented the need.

Mulling those two over, but 6000 RPM seems really really slow.

Consider a rifle with a muzzle velocity of 1000 feet per second and a one turn in 12" twist.

1 in 12 twist is one turn per foot.
1000 feet per second yields 1000 RPSecond
1000 RPSecond = 60,000 RPM
Even the stubby bullet out of a 1911 runs over 38,000 RPM. (16" twist, 850 f/sec)
60,000 X (12 ÷ 16" twist) X (850 feet per second ÷ 1000) = 38,250 RPM for the 1911 .45ACP RPM, if my arithmetic is correct.

EXTRA CREDIT:
Calculate the bullet RPM for a rifle firing it at 2900 feet per second with a 10 inch twist.
Hint:
Since this twist is faster than 1 in 12", the 60,000 RPM must be mutiplied by 12/10)
Since this velocity is faster than 1000 f/s, the 60,000 must be multiplied by 2900/1000
Thus:
60,000 X (12 ÷ 10) X (2900 ÷ 1000)

Back of book answer is 208,800 RPM

It is not out of line for a high velocity rifle bullet to hit a quarter million revolutions per minute.

For a long streamlined projectile, especially with the weight in the rear, 6000 RPM seems untenable.

Terry, 230RN

NOTE: The "Greenhill Formula" was only an approximation of the ideal twist for a given weight and diameter of bullet. The above numbers are not necessarily meant to imply "ideal" twists, but merely find the RPM for a given velocity and twist rate.

6000 rpm is a reference to the cyclic rate of fire of the Vulcan, not the numbers of revs in the bore. The Vulcan was designed to dump a swarm of lead FAST, due to extremely short time on target of its parent platform, the F-15 Eagle (I worked on A/B's and later C/D's). Another feature required to hit that high rate of fire is the use of electrically primed ammo, which eliminates traditional percussion ignition mechanisms used in other systems.

I worked on many M61A1's in the USAF/ANG, and they had maintenance intervals based on either round counts or time. If they didn't shoot, they stayed in the jet until they hit 18 months, at which time we pulled them for cleaning, inspection, lube and reassembly. Fighting corrosion was a constant battle due to aircraft flying through all sorts of adverse atmospheric conditions, and depending on the AOR, they would come out looking pretty rough.

Sorry, I had no idea that round count per minute had anything to do with the subject of gain twist versus constant twist.

I therefore assumed that "6000 RPM" meant revolutions per minute and was some sort of typo.

I still do not see any realistic advantage to gain twist rifling except possibly with arming method or driving band stripping, but I don't see that, either. With straight rifling, he projectile engages the rifling at, essentially, zero feet per second and therefore essentially zero RPM and both increase "in step" as it travels up the barrel to the muzzle velocity and RPM.

In addition, for non-driving band projectiles, it distorts the bullet more than straight rifling because of the different angles at which the rifling engages the bullet as it goes up the barrel.

(material deleted)

Terry, 230RN

20mm projectile rotational speed:

Twist = 1 turn in 20 inches, or 1 turn in 1.666667 feet.

Muzzle velocity = 3,410 feet/sec

1.666667 x 3,410 = 5,683-1/3 revolutions per second, or 341,000 rpm.

If you want to play with the Greenhill formula and the 20mm, the projectile is 3.428 inches long, 0.786" inches in diameter, and weighs 1543.25 gr,

I agree with your 341,000 RPM. But I inserted that note about the Greenhill Formula specifically because I did not want to discuss it.

I have struck out my estimates of angle versus twist above pending reconsideration of the method.

Terry, 230RN

230RN said:

In addition, for non-driving band projectiles, it distorts the bullet more than straight rifling because of the different angles at which the rifling engages the bullet as it goes up the barrel.

Rifling angle in degrees = tan^-1 (1 ÷ (twist at that point in the barrel.)

For ease of visualization and save you the calculation, the twist angles for various sample twists are:
One turn in 14" = 4.08°
One turn in 12" = 4.76°
One turn in 10" = 5.76°
One turn in 7" = 8.13°

Terry, 230RN

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Your twist angles are in error.

The circumference of the bore (land diameter times pi) divided by the twist rate (1-in-X) is the tangent of the twist angle.

For a 20mm with a 1-in-20 twist

20mm circumference = .786 x pi = 2.473"

2.473 / 20 = tan (twist angle)

Twist angle = 7.05 degrees

For 30 caliber (circumference = .9424"), a 1-in-7.62 will give a 7.05 degree twist angle

For .22 caliber (circumference = .691"), a 1-in-5.59 will give a 7.05 degree twist angle.

For a 105mm cannon to have a 7.05 degree twist angle, the twist rate would be 1 turn in 25.4 calibers, or 1-in-105 inches.