I'd be more concerned about losing accuracy, but bore diameter is only part of the equation. Bullet pull will be less since the case isn't sized enough to hold the bullet tightly, which can cause erratic ignition and that's where things really go south. That said, you might be able to get away with this using jacketed bullets but it's definitely a bad idea with cast bullets, which should be about .002" OVER bore diameter for best results. Undersized cast bullets lead like crazy and are wildly inaccurate, so they're basically a waste of powder and primers.
Okie John
