A little on Gaintwist rifling
Progressive twist is the technical term, "GainTwist" is also a trademark.
BTW,
The GainTwist Barrel Company
113 W Yellowstone Ave
Cody, WY
(307)587-4914
(307)527-6097 fax
still makes these types of barrels.
1. I can say with certainty the US 57mm Recoilless does not have a gain twist barrel (at least mine doesn't). As has been pointed out, most US recoilless rifles use projos with pre-engraved rotating bands for technical internal ballistics reasons. Yes, you have to line up the projos with the rifling when you laod the guns. For reference, the M18/A1 57mm Recoilless Rifle has a twist of 1:67.2". And the guns are designed to counter any torque reaction from imparting spin to the projos, hence they are "twistless" to the shooter as well as recoilless.
2. How much powder energy goes into rotating a projo? Do the math:
.50 BMG, 3000 fps, 1:15" twist, 700 grain projo.
Muzzle energy (kinetic energy in direction of travel) = 14,000 ft-lbs.
Rate of rotation at muzzle works out to 144,000 rpm.
We need to know the rotational inertia, but I'll simplify and assume that the projo is a perfect cylinder. Since its mass distribution is actually much closer to the rotational axis, this should be a close or conservative estimate (real projo rotational energy would be lower than this estimate).
Still working in the awkward Imperial units:
Mass of Projo = .0031 Slugs(no pun intended!!), Rotational Inertia = 7.3 x 10e-6 Slug-Square Feet.
Kinetic Energy of Rotation = 83 ft-lbs.
So the rotational energy of a projo, in this case, is about 1/2 a percent of its linear kinetic energy. And since perhaps 25-30% of the powder energy actually goes into the final linear kinetic energy of the projo (stoichiometric efficiency), I wouldn't call about one-thousandth a "substantial portion of the total gunpowder energy".
3. Yes, other military guns *do* use gain twist rifling. The 20mm Vulcan ends up at 1:20". The 40mm Bofors L/60 goes from 1:45" to 1:30". My Vulcan and Bofors barrels look kinda weird when you gaze down the bore, almost dizzying... I seem to recall bigger arty may use gain twist as well.
4. I believe the main advantages of using gain twist may be:
-reduced chamber pressure, from reduced projo start force from trying to make it instantly fit into twisting rifling
-less stress on the rotating band
-perhaps easier to manufacture twist that's constantly increasing, instead of worrying about backlash in the barrel rifling equipment causing the rifling twist to "drift"? This kind of drift may contribute to projo "balloting" as it "rattles" axially. Better to keep a rotational force constantly in the same direction?
5. Certain revolver cannons used smooth bores, with the only rifled portion a small insert just ahead of the cylinder. The smooth bore flattened down the flanged rotating band, to streamline the projo (much like Probert rifling). See the original Marquardt 20mm cannon, which evolved into a more conventional design (the Mk 11). I have a round of original Marquardt 20mm ammo, the wide projo flange also helped keep the round's case neck from crushing forward as it was rammed into the cylinder during the loading cycle.
-Phil