OK when cleaning this new 303 barrel after the function testing a few slivers of lead came out of the bore so obviously some leading was occuring(10 rnds). You did say that they were likely to be too small for the bore and you were right. This morning I oiled the bore and squished a 0.358" soft lead bullet down in diameter some then hammered it into the muzzle and got a nice formed plug of the first 0.320" of the bore. We know that the land diameter is a smidging under 0.302" as it leaves trace marks on the 0.302" front portion of the cast bullet so it's about 0.301" and it appears that the groove diameter is 0.315". This was measured by roling the bullet round on the anvil of the micrometer and the reading taken that just allowed the bullet to rotate (Five lands and grooves).
This means the groove depth is 0.007" and my commercial cast Gas Checked bullets are 0.004" undersize which is never going to work without leading .
Now do I save up and get a custom mould or would using 0.308" cast bullet and paper patching be an idea? .......... Or get a mould for the 32-20/32-40 (0.321") and size it down the required 0.005". I tried looking for an early 8mm mauser mould as they would be 0.318" but failed to find one .
Following this I guess I will have to get either the Saeco or the Lyman 450 lubesizer working and the proper dies for it/them.