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Check out David Southall's articles on .303's with oversize bores here:

Cast Bullets in the Lee Enfield Rifle

While I haven't had the tumbling problem David Southall says that fillers can help fix that.

Also, most moulds for .303's seem a little undersize for old military bores but at least one outfit makes moulds specifically for oversize .303's:

CAST BULLET ENGINEERING Bullet Moulds Molds

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