Rifling question

[KJS]

I know different bullets require different amounts of twist to stabilize them. That's about all I know.

Why do some guns that fire the same ammo have different rates of twist and a different number of groves? Seems especially odd when the examples I've selected below are all from Ruger and fire the same .357 ammo.

-GP100 & SP101: 5 groves 1:18.75" RH

-Blackhawk & Vanquero: 8 grooves 1:16" RH

 

[Jeff82]

Heavier bullets (longer) need faster twists to stabilize. Looks like the Blackhawk & Vaquero were intended to fire a heavier bullet. Number of grooves is just the choice of the manufacturer.

http://kwk.us/twist.html