I just got a .308 Marlin Encore Pro-Hunter 28" fluted SS barrel at a smoking price. Called MGM to see what I could rechamber it too and it looks like a .30-40 AI or any magnum like .300 H&H, .300 Win, or .300 Weatherby for $75. The .30-40AI has me running right along with the .30-06 from what little data I've found.

According to Garry Reeder's site the .310 GNR puts me between a .30-06 and .300 Win, so I'm thinking similar to a .300 H&H. All I know is I don't want a .308 Marlin, but I couldn't pass up a $130 barrel NIB.

I reload the .30-40 as well as the .300 H&H so if I don't go for the AI version or GNR, I'll probably go with one of these two standard chambers. The reason I'm even considering the .30-40AI is from reading Ackley's books and recalling it was one of the better performers.

Well I learned that I can't clean up the chamber with any of the original three choices. Looks like I have to find somehting else to rechamber to.

Where did you get the barrel?

I am not sure on what you mean by CLEAN UP? Yes the 30-40 or AI can't be done but my drawing of a 308 Marlin show it with a .470" web and and a .455" shoulder 1.460" from the base. Am I wrong. If it is true then you could do any 30 cal case with a 06 or Belted Mag (30H&H-300 Win-300Weath.) base that is wider than .455" 1.460" from the base. The 300 H&H is .462" 1.460 from base and the Win and Weatherby are .500". As far as I know all of the after marked Encore barrel makes will not chamber for the WSM cases or the larger web sizes dia's which leaves out the 300 WSM and the 300 RUM.

If you really wanted to get way out there you could try the 30R Blaser, but then you would be wedded to an oddball, proprietary Euro chambering.
30R

Another rimmed choice would be the 307 Winchester.