John,
The "common wisdom" is that when you start with an as-cast .312-.313" diameter bullet and size it down to .309" that you will lose accuracy.
Maybe for the most precise of cast bullet target shooting you will, but not for the majority of us that want the single bullet mold to fit several .30/.303 caliber rifles. For plinking or even hunting use, it should be fine.
I've sucessfully loaded .303 British, 7.7 Jap, .30-40 Krag, .30-06, .30-30, .308, and a host of other .30/.303 cartridges using the same mold. The Lyman 311291 at 170 grains was particularly good for me and a couple of shooting friends.
The one problem I've found is that on some long bore-riding bullet noses, the .312/.313 size bullets tend to be too large for the .300 bore of standard US .30 caliber barrels. This can be be fixed if your chamber has an extra-long throat or bullet seat.
You may find an occasional barrel with oversize groove diameter (.314-.317), and the one-size-fits-all approach will not work.
John