wookiemeister said:
I think I’ve seen somewhere some shells are designed to deliberately tumble, so was thinking
Projectiles are not designed to ‘tumble’ in flight.
Once they start doing that, any hope of accurate shooting goes out the window entirely.
What you’re probably referring to is the M193 5.56mm X 45 mm NATO cartridge. This was the original cartridge for the AR-15/M-16 rifle. That rifle originally had a 1-turn-in-14-inches rifling, which gave the M193 a relatively slow rate of spin.
That rendered the M193 only just stable in flight, particularly in cold air temperatures. When the bullet hit the target, it lost all stability, and it tipped over and slammed bodily into the target. As you can expect, this is bound to produce a much more severe wounding effect.
But, it was never supposed to tumble in flight
The slow spin rate didn’t do anything to help with accuracy, as the bullets would tend to yaw a bit after leaving the muzzle and the rifling was revised to 1-in12, and later to 1-in-7.
The later M855/SS109 cartridge had different propellant, and was considerably more stable in flight. However, both the M193 and M855 are meant to fragment on impact (otherwise, you’re just punching .22 size holes in things).
The M193 was not necessarily a bad cartridge, and it functioned quite well in early Ar-15s, M-16s, and M-16A1s. Indeed, it was far less likely than the M855 to cause cartridge feed problems, but that’s to do with gas pressures in the chamber and at the bolt face.