There’s no teeth on the shell/ bullet. The outside diameter of the round is larger than the nominal bore/ calibre (which is measured across the smaller diameter that can be seen – ie across the “tops of the teeth” in the barrel). The driving band ( bourrelet) or jacket (on small-calibre bullets) deforms is of a sufficiently large diameter to fill the gaps to the “bottom of the teeth” – which is what leaves the striations that police forensic scientists look at when they do a ballistics check.
There’s no need to line up anything, the outside diameter is “perfectly” circular.
Typical bullet actual diameters are not what they’re called eg 5.56 (NATO SS109 round) is 5.656 mm, 20 mm shells are actually about 19.9 but the bourrelet is large enough to fill the gaps (sorry I can’t provide more figures, I’m not at home and all my note books are 🙁 )
The only calibre I can remember that actually has a bullet diameter that is what it’s called is 9 mm Parabellum, which also happens to weigh ~9 grammes.
Tony Williams’ site – http://www.quarry.nildram.co.uk/ will have information of that sort, and he usually posts on here as well. I recommend his books, they have (OK I admit I’ve only got the autocannon one so far) huge data tables at the back with that sort of data.
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