I've never been much on cast bullets in rifles, but when I did quite a bit of casting for handguns I found many seemed to shoot better with the bullets just slightly oversized. Most other folks swore by bullets sized to the exact measurement of the grooves. But then there was different alloys and others issues to worry over
I cut my teeth on reloading cast bullets in my Springfield- probably the easiest rifle on earth to shoot cast bullets through. Mauser 98s are easy too, if you can find a mould big enough (another one of those rifles where groove dimeters are much bigger than advertised, but that's another story) on both I size bullets to be .001" over groove diameter, seat the bullets to engrave in the rifling, and give them a charge of about 18gr of 2400.
The swedish mausers were a rude awakening. Not only is it difficult to find a mould large enough in diameter (hence the custom bullet mould orders, there's even a custommould sold by Midsouth that casts at .270 called the 'cruise missile') but the barrels have a very tight twist rate and makes for finicky shooting with cast. I've tried a .265" sizer for my tighter Swede bores, but it just won't shoot and I get a ton of leading. Having the base of the bullet at .268" allows the bullet to swage into the bore.