I am considering building my own rifle, either a 7mm or 30 caliber. What would be the best barrel twists for each of these calibers? I wantthe rifle to be good for everything fromantelope to moose. I would shoot a 140-160 grain bullet in the 7mm and a 180 in the 30 caliber.My choices for the 7mm are 8, 9 1/2, 10 and 12. 30 caliber are 8, 10 and 12.

What cartridges?

I can't speak to the 7mm, but a 30/06 has generally a 1:10 twist and is accurate with a wide range of bullet weights
It seems to me that a .308 has a 1:12 twist and ismore accuratewith lighter bullets.
The slower (longer) the twist rate, the shorter the bullet will be stabilized.

Spencer - the caliber actually matters very little here. Generally, you are best off with a 1:12 twist for a .308 caliber. It is optimum for accuracy for bullets in the 150 to 180 grain range, and will stabilize most 200 grain bullets; and I really don't see much need for heavier pills - particularly in this day of bonded bullets.

In the 7MM, a 12" twist might not quite work with some 160's under bad conditions (cold dense air, high humidity,low elevation); so I think I would err on the side of stable and get a 10". I would order an 11" if it's available, but not everyone makes one.

IMO the twists to get are 1-10" for .30 caliber and 1-9" for the 7MM

The comments previously posted are correct and good....however I've never barreled an action with too fast a twist....but I have used too slow a twist.

I error on the side of faster.

I like to error on the side of fast twist also. You can't overstabilize a bullet unless you are shooting very high velocities (3800+ fps) and using very frangible bullets which may come apart. But this is generally a problem with very fast .22 centerfires like the Swift, not a 300 or 7mm magnum.

You can figure the twist you want by using this formula.

Twist = 180 X D squared / L

D = bullet diameter in inches
L = bullet length in inches

Actually it is nice to know the velocity range.The 1 in 12" usually works very well for the 308win and other lower velocity cartridgesbut the 300 magnums seem to do better with a 1 in 10" twist.

I like the 1 in 10" twist for my 7mmstws and for the 7mmremmag,but some people prefer a 1 in 9" twist.

Too slow a twist can result in the heavier bullets not stabilizing properly and a tighter than required twistcan reduce the velocity potential slightly.Lazzeroni uses a slower rate of twist claiming that it provides more velocity with equal pressure.There is one more alternative which is the gain twist rifling where the twist rate actually changes in the barrel.Gain twist barrels are said to provide more velocity with equivalent pressure.

For the .30 cal 1-10". For the 7mm 1-9.50". Good luck.

The first part of this statement is absolutely concrete...it can't be argued.

The second part however is theoretically true only. I've never been able to measure it and I'm beginning to think the difference is less than the shot to shot variation of a given load in a given barrel and twist. In other words...it's marginal at best.