.312 bullets in a .311 bore?

[Jack1939]

is... asking for trouble? no problem because going 0.001" over is not a big deal? I have some of the .312" hornady 150 grain bullets and a mosin that has a bore diameter of .311" and am wondering if I should try to work up a load...

[noylj]

It really doesn't mean a thing. In fact, I have never had a problem with 0.312" bullets in my 0.308" Win M94 barrel. You always work the load up--never start at mid-range or higher.
If you have slugged your barrel and KNOW what the actual groove diameter is, you can better select a bullet.
Jacketed bullets up to 0.002" over groove diameter REALLY doesn't matter at all--but a jacketed bullet exactly at groove diameter or 0.0005" larger will generally be the most accurate (it might take as many as twenty 5-shot groups to establish any actual improvement, so it isn't that significant except at high levels of competition.
The thing is that so many ask this question and NEVER check their barrel--they assume that their barrel is EXACTLY at SAAMI nominal dimensions, or even a bit tighter since it is their gun and must be near perfect.

[Cowboy T]

Actually, if you're using cast bullets, you're supposed to go 0.001" over. Since I cast my own, I actually like to go with the largest cast "boolit" that will physically fit, i. e. the rounds chamber reliably.

If you're running absolute max loads and running jacketed bullets, just watch for signs of pressure. And as noylj pointed out, you always work the load up from the starting loads.

Even jacketed, you'll be just fine with a 0.312" bullet through a 0.311" barrel.