[Ulzgoroth]

If you look at the numbers, they aren't actually that weird.

5mm and 7mm are obviously truncations of the most common modern rifle calibers. The reduction isn't negligible, but it's only around 10%. I'd be surprised if you could find those in black powder, I'd think something so small would work terribly without smokeless (EDIT: actually, 7mm I guess you could, weren't the first ones smokeless adaptations of final-generation black powder cartridges?). 10mm is an actual modern pistol caliber, similar to some other modern pistol calibers, and also a close match to some big-game rifle calibers (which go back to the 19th century and carry forward to the present). 15mm is square-on for the caliber of US Civil War era rifle-muskets (the Springfield and Enfield anyway) and very close to the heavy soviet 14.5mm HMG/AMR caliber.