big,
You can use any lump of lead that starts out bigger than your bore for slugging. Just drive it in. The excess will shear off at the muzzle with no harm to your bore. I've use hard 45 caliber slugs to slug 30 caliber rifles before.
If your two bores are differant (and it's likely) you may be able to size large for one and larger for the other. For instance, say they are .4291 and the other is .432. You probably would have no trouble sizeing for both at .432 or even .433. As long as the bullet will chamber, you're good to go if you're over the bore size.
Welcome to the wonderful world of cast bullets!
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