After shooting various cast bullet styles/diameters through .30cal rifles for over 30yrs, I've settled on .311" for ALL my .30 cal's.
I started using .311's when working with some .32acp cast 78gr RN and Bullseye powder with a .30/30. at 50' I was getting 1-hole groups and at 50yds was getting far better accuracy than I'd ever seen from this particular rifle, to include jacketed bullets.
I scratched my head and cast some 150gr FNGC (Lee .308 FNGC, cast to 160gr) sized/lubed them to .311", and loaded them over my typical load, 28.0gr of IMR3031 at the time. Viola !! near MOA groups at 100yds w/scoped rifle. Now I use RL15 for equal or better accuracy and even higher velocities (2,200fps hunting load).
I've also used them in several .30/06 with equally accurate results....
Like another poster mentioned, if the bullet will readily chamber after loaded into the brass, it'll shoot fine. In the matter of .30/06 and .308 it's possible that some mil-surp brass may have some overly thick necks that could cause some problems. I also saw this with some PMC brass in a 7mm08. I neck-turned the brass and "fixed" that problem...
For neary 20yrs, I'd followed the Lyman advice of 0.001" over bore with cast bullets; ergo I used .308 and .309" sizing and "accepted" the results w/o questioning the recommendation.
I've used as large as 0.313" in the .308's (.30/30, .308, .30/06) and as long as they chamber easily, they're fine. I recently loaded for a friends .303Brit. I started with .314" and decided to try some .311". They too shot fine, so I only use one size for simplicity, 0.311. They've even shot very well through a Moison-Nagant.