SmallCaliberCentral

Anyone any idea how much velocity I could expect to lose going from a 1/12 twist 24in barrel to a 1/10 twist 24in barrel with 40 grain bullets ?

probably not enough to worry about. You will be happier with a 1:10 twist for 40 grainers in most cases anyway. My best guess based on just playing around with my guns would be about 25 fps and if your 1:10 helps you obtain greater accuracy then fps loss doesn't matter.

I agree with Ryutzy. You won't loose enough to worry about. Not to mention that the extra RPM from the 10 twist barrel= extra carnage when you connect with a target. We were running a Browning A Bolt 204 with a Shillen 10 twist barrel this summer while shooting prairie dogs in Colorado. We noticed an increase in carnage with the faster twist barrels.

I've seen this "discussed" in a number of different forums but I never have seen an "empirical" answer offered. So I'm going to swim upstream and say you might gain velocity.

Further confining the bullet increases the pressure behind it. More pressure = higher temperature (ideal gas law). Under greater pressure and higher temperatures, the powder burns faster than if the pressure were lower.

A lot of people who shoot moly-coated bullets are surprised to learn they lose a bit of MV with a given charge weight. This is the same principle in reverse. The slicker bullet doesn't need as much pressure to reach the same acceleration. The powder isn't as confined so it doesn't have the compression it needs to reach its optimum burn rate.

Worst case, if there is some loss of MV, I don't think it will be very consequential.

The faster twist will gain stability in Heavyer bullets. Spins them faster for more stability. Velocity wise you go slower with heavier bullets. However, Often in a long range situation they shoot much flatter. They also make a better thump on impact? I shoot a 1:9 twist Shilen in 204. I have to say I wonder If all the research was done when recomending the 1: 12 in 204 bye Ruger. Did they just release what they had To get the sales going?? I sometimes wonder after testing loads in my 1:9 The fast twist barrel is a awsome shooter, and not just mine. I helped a friend build the same and Ditto!