It makes sense in terms of the sophistication required to make them- plasma flasks are just containers of pressurised hydrogen, melta canisters are bottles of pyrum-petrol fluids, but bolt rounds are sophisticated little rockets. Much harder to construct.
All that said, my rule is the team's inquisitor will ensure they have a minimum of 1 full magazine of ammunition for whatever weapon (or, in the case of weapons with a capacity under 8, 2d5 shots) they use, for free, with any extra ammo costs being absorbed by the PC. I do this almost solely to make bolt weapons more attractive, and to provide a safety net for Scum.


