Originally Posted by Nitroman
It will be 24 inches in length, as large a diameter as will fit under Millie's own free-float handguard and through his Picatinney gasblock.
I did the same thing some time ago so I will offer this for your consideration... I found that the rifle was front heavy (just plain heavy!) and only suitable for shooting off a bipod or rest. It was however, extremely accurate. If you plan on carrying it much while hunting, I would suggest a thinner diameter barrel to reduce the weight (might even consider fluting it) and put a heavier stock on it, like the Magpul PRS. It will help the balance a lot.
As far as the twist rate is concerned, you can shoot pretty much anything you want to include the Hornady 75g BTHP. You probably won't be able to use the 75 AMAX out of the magazine because of the length of the ogive. It causes the bullet to be seated a little below the top of the neck, however I would expect that it would work well single loaded. The 77g SMKs should work too.
I would listen to the 'smith about the chamber dimensions very closely. AR's will generate a lot of fouling and that fouling will find it's way into the chamber. If you have too close a fit it will cause malfunctions. One more thing, Never, Never neck size the cases. That WILL cause you problems.
Good luck on your build. Hope you have a great time with it.