I'd go with the .311. (Have, in fact.) If .309 ain't big enough, .311 won't be too big, and .310 just might be too small. You also make a good point about GCing and lubing bullets that have already been heat treated. A thou over might make a difference in a benchrest gun, but you ain't gonna be able to see it in a lever Marlin. A thou under, you can sure see.