The best bet for a 1-12 twist is 45 to 52 grains. Also, you will note that flat base bullets group better at 100 yards than boattails. However, what groups you net at 100 yards with a boattail, you should be able to maintain out to 300 yards. Specifically I mean that, my Rem 700V in 6mm Rem grouped 3/8" at 100 yards. The rifles also consistently grouped 3/8" at 200 yards, and 3/8" all the way to 300 yards where I set my zero. In my experience, the group size did not open past 100 yards. We noted this same fact with all of the varminters in our group, and in all calibers, when using boattail projectiles.
Sierra makes about the best projectile I have ever used. They have consistent measurements from projectile to projectile, and thus they group the best.
Edit to add a caution: Do not get hung up on the elusive single-hole cluster of 5-shots. It's silly, and a worthless waste of money in most 300 yards or less varmint shots. If your rifle is grouping 1" or less at 100 yards, you will nail 99.99 of varmints out to 300 yards.
The greatest knowledge and ability that the varmint hunter needs, is to know without exception where that 1st shot out of a cold, clean barrel will impact. That is my experience over the more than 20 years that I have hunted vermin.
Other folks may disagree, and that's fine. But for me, I will not dump $2,000.00+ into a varmint rig to reduce to that degree of grouping. Now, if you are varmint hunting with a .25-06 Rem out to 500 yards, that's a different matter. But, at 300 yards and closer, which is where one uses a .223 Rem, 1" or better at 100 yards is good enough.
Here are some pics of the way I have my Rem set up:
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