If say a 1-12 with a 50 gr at 2700 produces 100,000 rpm and the bullet takes x amount of time to reach target producing 300 revolutions in that time.
Now we up the velocity to 3400 fps producing 160,000 rpm and x amount of time to get to target but it will still only spin 300 times from barrel to target, its not going to spin any faster because of velocity. well yes and no.
Yes, the bullet spins faster because RPM's is revolutions per minute which is a measurement of time, not distance.
so how does velocity stabilize it more ? it just gets there quicker with the same number of spins
The bullet is more stable because as the the RPM's increase the centrifugal force around the axis of the bullet increases. Think about a top spinning. The higher the RPM's the more stable the top becomes even though it isn't moving over distance any faster.
The bullet will still spin the same number of times over a given distance. That is always constant due to the barrel being constant. Excluding outside forces like air resistance which create friction on both the spin of teh bullet as well as the velocity of the bullet.
As velocity increases the RPM's increase. So, as velocity increases you will not need as high a barrel twist rate to stabilize the bullet. If a bullet needs 150,000 rpm's to be stable a barrel twist rate of 1in12 will accomplish that with 2500 fps. If the velocity is increased to 3000 fps you only need a twist rate of 1in14.4. The bullets rpm's will remain the same even though the bullet will spin less over distance.