The effect of the spin from the Magnus effect is seen with golf balls, baseballs, and lots of other things. The idea in gunnery is that a bare ball is given spin by contact with the barrel and one uses just enough bend to make the top of the muzzle the last point of contact of the ball with the barrel. It has the effects I mentioned:
- It gives the ball a consistent spin, eliminating the variability of spin from random contact with the barrel or from the knuckle-ball effect on an initially non-spinning ball.
- It orients the spin so the effects are in the vertical plane of the trajectory, with no lateral deflection.
- It produces back-spin, which gives the ball lift, and thus a flatter trajectory.
This was first seriously investigated by Benjamin Robins, and described in his New Principles of Gunnery in 1742. As cited at
http://www.napoleon-series.org/military/organization/c_velocity.html: "Robins also investigated lateral deflection of high-speed projectiles. He set up a series of evenly spaced paper curtains that allowed him to measure the enormous deflections of a musket ball in flight. In one test a ball measured over a range of 760 yards deflected more that 100 yards to the left. Robins identified that it was the spin of the ball that caused this deflection. In a further experiment he bent a gun barrel a few degrees to the left. Although the bullet initially moved towards the left, eventually it reversed its later direction and crossed to the right of the barrel (this effect, which can be observed in baseball and tennis balls, is known as the Magnus effect)." I don't have the reference any more, but I'm fairly sure I remember him specifically writing of the possibility of this muzzle-bend in a "gun" (i.e. smoothbore in British usage) intended for ball, and of this being used by gun makers back then, just as this effect is used today in airsoft guns. Obviously, this bend will throw the pattern low if shot is fired from the barrel.
If a ball is fired without spin, it appears to be essentially a knuckle ball and eventually break in a random direction, just like a baseball. Patched balls appear to be commonly launched without spin, while unpatched balls may or may not be, depending on the specifics of wadding and loading technique. The important questions for a shooter are how far downrange does the ball get before it starts to break and how fast does it diverge after it starts to break. Concerning the first aspect, many smoothbore shooters have reported getting their best accuracy with patched balls, usually tightly patched, at comparatively high velocities, so I suspect that suspect that pushing the ball hard gets it further downrange before it starts to break.
Another possibly related factor is the turbulence encountered at transonic velocities. This factor affects all projectiles and is significant enough that modern long-range shooters seeking the best accuracy try to use a cartridge/load that will keep their bullets supersonic all the way to the target. Here again, a higher velocity will move the subsonic transition further downrange. This turbulence may affect the onset and/or magnitude of the knuckle-ball break, but even if it does not directly affect the break, it will compound with it to increase dispersion (and wind drift IIRC) at ranges beyond the transition. This is also why those seeking the best shorter-range accuracy may choose to use a muzzle-velocity below the speed of sound - match-grade .22RF ammo, for example - to avoid this turbulence.
I haven't seen enough comment about the second knuckle ball aspect - the magnitude of the break - to make any observations concerning the effects of velocity.
Regards,
Joel
