[Brian Pfleuger]

I believe that you generally want lead bullets that are .001 OVER sized. I'm not a lead shooter so I'm not positive but that's the number I usually see.

As for shooting a 300gr bullet, there's nothing inherently dangerous about it as long as you do it right.

You'll need to make sure that the bullet isn't jammed into the rifling and can be seated to a depth that allows for an appropriate powder charge to be used.

You may have trouble finding data for that bullet weight so you might have to experiment on your own. Needless to say, doing so can be extremely dangerous if you don't have a thorough understanding of what's happening.

I assume you are new to reloading and I would suggest getting some experience with "normal" practices before venturing out on your own.