- using very fine abrasive to ensure that the inside of the mounts are perfectly round and exactly the same diameter as the tube (they certainly won't be on mounts that cost £30 - £50).

You can using fine grinding paste on a length of 30mm steel bar (ideally), or you can stick a bit of e.g. 1000 grit wet-and-dry to your scope and rotate that in the mounts (see http://www.bloom.co.uk/images/sportsmatch_honing-s.jpg)

Warning (1): the latter method will inevitably tend to over-bore the mounts due to the thickness of the paper, so proceed with great caution. The "lapping bar" method is superior - if you can find a piece of perfectly round 30mm bar . . .

Warning (2): doing any of this is regarded as eccentric/daft by most airgun owners