Crankgrinder,
Your worrying about oversizing too small to matter. The military, for example specs .30 cal rifle barrels .3065"-.03095" groove, and bullets .307"-.309". So you can get a .3065" bore shooting .309" bullets and there isn't even enough performance difference to affect the sight settings. And that's in high power rifles where pressures are already a lot higher. Then you get into the Euro-30's and their various diameters from .308" up to .314" (with some even bigger exceptions), and people cross bullets between them frequently. I've got a French MAB with a .309" groove barrel for shooting the standard 0.312" .32 ACP. No problem, and that's a jacketr bullet in a fairly light barrel.
When you get to cast bullets, they are usually soft enough to shoot an even further oversized bullet than the jacketed ones above. For years, .45 Colt revolvers were chambered for the old .454" diameter ammo, but sported a barrel in .451", so as to be compatible with both bullet diameters (though the extra loose chambers don't help accuracy). The main reason not oversize your lead bullets too far is accuracy. Lots of guns, lever guns in particular, and many revolvers, seem to shoot better with cast bullets .002" over groove rather than the usual commercial .001" over. However, once you get to .003" over groove, many seem to start to have their groups open back up. Not sure why .002" is such a special number in them.
Anyway, you're nowhere near a danger zone. If these were 0.010" or more over, I'd start showing some concern. And that wouldn't be because of pressure so much as concern that if a bullet that fat will chamber in your gun, there is likely a chamber dimension problem with it.
P.S., I know a lot of folks measure slugs with calipers, but it's easy for them to be off a thousandth or even two, sometimes (though the modern digital ones do surprisingly well at this point). Anyway, if you haven't got one, go to Harbor Freight or Enco and for around $20 you can get an
OD thimble micrometer with .0001" Vernier scales. These are much preferred for measuring slugs both because of the greater resolution and precision, but also because the wider measuring surface gives you a more precise feel on touching down on the slug. Using a
cheap micrometer stand makes it even better.