sqrlgtr,
you could do a lot worse than building a 6mm Ackley Improved for light bullet usage on varmints!
I don't know why a 6mm-284 was mentioned so much, when I saw no mention that YOU were interested in it. And for the record, the 6mm Ackley Improved and the 6mm-284 are not even close, at least with the many rifles I have seen in both calibers...
If you are seriously wanting a 6mm A-I for a varmint cartridge, you have made a very fine choice! 55 gr. bullets will very easily exceed 4,000 fps. and 70 gr. bullets will be right on it's heels...
Lilja barrel / 3-groove / 1-13" twist / heavy varmint contour (minimum) / 26" - 28" length
If you get a barrel that has a 3" to 5" straight shank section at the chamber end, when accuracy goes to pot, you can cut off 2" of barrel, rechamber, cut an inch off the muzzle and recrown, and should get a minimum of a thousand more accurate rounds through that barrel before it becomes a tomato stake!
I am speaking from experience, as I have done this many times and find it do be very worthwhile...
Barrel life will depend more on how you take care of your barrel than anything else. Don't let it get scorching hot, and when you are done shooting for the day, right after the kast round has been fired, coat the bore with a 50/50 mix of Shooters Choice and Kroil. This will keep copper build up to an absolute minimum. Also, run a wet patch down the bore every 3 days, and keep the bore "wet" until you get ready to go shooting again, then dry it out good, just before you go shooting again. Also, NEVER use a brass brush when cleaning your bore. Always use nylon. With your barrel properly maintained, your first shot from a cold barrel should be dead on...
If you have a hankering for a 6mm Ackley Improved, then by all means, build one and enjoy it!