Why is a 1:10" twist necessary in my .308 Win rifle? I looked at a twist rate chart and it would appear that 1:12 would suffice.

Looking at a chart (or reading stuff on the Internet, etc.) does not always get you where you want to go. The Subsonic World is a whole different realm. The distinction was best explained to me long ago by the famous barrelmaker and ballistician, Boots Obermeyer (who has done projects for Uncle up to 30mm). At Subsonic velocity, you have not only 1/3 of your forward velocity, but also 1/3 of your *rotational* velocity. Think RPMs. It is therefore more difficult to stabilize a subsonic projectile than a high velocity projectile of exactly the same BC, profile, etc. Ergo, you "spin it up" to assist in this stabilization process. The reason EBR recommends 1:10" for use with both supersonic and subsonic velocities is that this twist rate is sufficient to stabilize a reasonable subsonic round, yet gets great performance with the commonly-used 168-175 gr. high velocity projos.