I've often wondered this too. Could it be the air passing over the driving band?? I know the grooves are quite tightly packed on a 77mm, but more spaced out on a French 75. Could that cause the sound as it spins? Not the most scientifically explained theory, and purely a shot in the dark!....
If subsonic, the noise is likely caused by the projectiles motion through the air, the rotating and engraved driving band interacting with the air, or a combination of the two.
I did some calculations using data for the 77 mm WWI-era German guns; so for 7,7 cm Feldkanone 16 (7,7 cm FK 16) and 7,7 cm Feldkanone 96 neuer Art (7,7 cm FK 96 n.A.).
According to the WWI-era UK data these have maximum respective rifling pitches of 7.15° (7° 9’) and 8.5° (8° 30’). These pitches, if my potentially dodgy maths is correct, equate to respective twist rates of one turn in 25 calibres (1:1.93 m) and one turn in 21 calibres (1:1.62 m).
Muzzle velocities for the 7,7 cm FK 16 range from ≈748 to 1,526 fps (≈228-465 m/s). So, this equates to spin rates at launch of ≈119-242 revs/s (≈7,140-14,530 rpm).
Muzzle velocities for the 7,7 cm FK 96 n.A. range from ≈350 to 1,975 fps (≈107-602 m/s). So, this equates to spin rates at launch of ≈66-372 revs/s (≈3,960-22,300 rpm).
You’d either hear the projectile spinning at its residual (*) spin rate, or this rate modified (modulated) by the number of grooves (32 for each of the above guns). The spin rate in rev/s would equate to the frequency figure in Hertz, or 32 times this, or more likely combinations of the two.
The sound heard would also be modified by Doppler effect as the projectile moves towards, or away from the listener. So its pitch would go up as it approached, and down as it moved away.
(*) Like the velocity, the spin rate decays with time, but not normally as quickly. These different decay rates can result in projectile stability problems due to over-stabilisation of the projectile downrange.
I found this
https://www.quora.com/Why-do-artill...rge bullets,friction and produces the whistle
One thing if you hear the shell you are Ok because it has passed you location. It is moving faster than the speed of sound so you will not hear the one that hits your location.
You wouldn't hear a whizz then, just one or two successive cracks. One would be the projectile functioning, the other the shockwave created by the projectile's supersonic motion through the air. The former has the potential for being rather louder than a crack! The exact times these two cracks are heard and which is first, second, or if they are heard at the same time, would depend on where you and the projectile are at the time they occur.
