The U.S. military used the M16 series rifle with a 1:12 twist for a couple of decades. The standard round used a 55 grain projectile. At some point they changed to the SS109 cartridge with a 61 grain projectile, and the rifling changed to a 1:9 twist. The heavier bullet needs to spin faster for stability.
How does faster rifling (1:9) affect a 55 grain bullet, ballistically? Does the faster spin accentuate any imperfections in the projectile, making it (potentially) less accurate? Or is there virtually (practically) no effect? Is muzzle velocity reduced and if so, by how much?
The lighter bullet will experience what is called “overstability” which means that its nose will continue pointing in the same direction even though the bullet is starting to drop along its parabolic arc. Or in other words, from the bullet’s point of view, it starts to go nose up from the direction of travel. This causes instability and inaccuracy.
What you really want is for the bullet to spiral like a football and keep its nose pointing in the direction of travel. The Greenhill Formula is a good rough approximation to calculate the ideal spin and twist:
Googling Greenhill Formula will give you a lot more details.
ETA: Diagram of overstability in case my description isn’t clear:
The mythbusters found that falling bullets without any ballistic arc (i.e. their noses were not pointed in the direction of travel) were not likely to cause serious injury.
This contrasts with the annual reports of people injured by falling bullets that somebody else fired into the air, usually around the 4th of July (like the schtick you see in movies and cartoons of people firing their guns wildly into the air in celebration).
They reconciled the two by noting that any angle short of perfectly vertical will likely result in a proper ballistic arc.
