I should add that it is commonplace to make 6.5x53R cases from .303 British, which is certainly doable but not trivial since neck-reaming should be done to prevent premature neck-cracking.
The step often missed though, is reducing the rim diameter by a few thou (from 0.540 to 0.530 inch) because the .303-based cases will still feed, chamber, fire and eject just like originals.
So why bother?
The problem comes loading the clips, which is achievable with unmodified rims but after a time will distort the clip and make it difficult to get a tight en-bloc loading.
These days we have to make our clips last for many, many loadings, so rim reduction is highly recommended despite the difficulty involved.

...and as Mike has pretty-much implied, very important not to step on the empty clips when they drop on the ground in front of you!