1 After some reading, it has a minor effect.  Though likely not enough over WW2-era battle tank distances.  Something like the Atomic Launcher, however, would have taken it into effect.

Though modern tanks have gone to Smoothbores due to the ammunition types they use.  As the cannon ammunition got longer & bigger, rifling simply doesn't add enough.  Now they just stabilize the rounds in flight with fins.

2 It's probably a matter of convenience. I would expect right-hand threads because that is the primary direction of rotation on a metalworking lathe.

3 sqa: It's more that rifling is actively harmful for the two major anti-tank types of rounds, and I'm pretty sure also for canister rounds, which are essentially big shotgun shells.

Sabot kinetic kill rounds fire a dart of tough metal with fins, as you note.  That apparently does a better job of stabilization than spinning, and the spinning induced by rifling adsorbs quite a bit of energy, so you get better velocities and therefore penetration if you omit it.

HEAT rounds use shaped charges, and spinning disrupts the penetrating jet they produce.

One wonders why rifled tank canon lasted so long.  In the West, in the late '50s the British developed the fantastic rifled L7 105mm canon which pretty much everyone adopted, and it e.g. provided a decisive advantage to the IDF in the "Dual for the Golan" in the 1973 Yom Kippur war (the battle I'm current closely studying, one of the greatest tank battle of history).

But by the time we fielded the first generation of the M1 Abrams, we were installing slip rings on the outside of our anti-tank rounds to negate the rifling effect.  Then we and a few others adopted the smoothbore Rheinmetall 120 mm L/44, first produced in 1974.


Hmmm, per Wikipedia the Soviets introduced the first smoothbore tank cannon in 1961 with the T-62's 115mm U-5TS.  The article on the canon said it was effective against Western tanks until the introduction of "third generation MBT's in the late '70s and early '80s".

4

I just received an email about this:

One concern in choosing the rifling direction arises if the design has a gun barrel that threads into its receiver; the rifling direction should be chosen so that the countertorque generated by spinning the bullet up to speed should be in tightening direction of this threaded interface.

The M3 grease gun got this famously WRONG, so that over time the barrels would loosen from their receivers, and if you didn't check for this regularly then after a LOT of full-auto bursts you could spin the barrel off, which could get rather embarrassing to say the least.

Back in the USS Clueless days, I made a couple of posts about tank guns: one two

5

Hmmm, I never ran across those articles about tank guns.  I would comment that the statements about why the Soviet tanks and equipment as designed as such are correct as far as they go, but are not complete, since there actually was a logical reason for this.  I would wonder about saying the T-72 was a 'low-end' tank vis-à-vis the T-80, though, given their respective design histories.

C.T.

6 @ hga: a "canon dual" sounds interesting.  What is it, two string quartets on stage at the same time?