Actually in the future ammunition is transmatted into the chamber and then shot out railgun-like at 90% of the speed of light. That's why all shots (other than rockets) appear to instantly connect with their target. The physical aesthetic of a cartridge and barrel was kept as purely cosmetic details. So while technically the ammo pack is in the cartridge you replace on reload, it doesn't have any physical connection to the firing chamber. And both are just holdovers from a history nearly forgotten. Since there was no actual engineering reason to have the barrel line up with the magazine, that part has been lost over the ages. Now rockets are a bit special. They're constructed to explode and firing them railgun style results in far too many explosions in chamber. So they are still propelled by chemical means when splash damage is required. Since they're that much more complex than dumb slugs, you can carry less in the transmat cartridge. All ammo works on principles similar to synth packs. That's why you can use the same generic archetypes (primary, special, heavy) for multiple different specific weapons. All the interesting stuff is in the gun. The ammo pack is just the go juice. Literally. The ammo packs are made of liquid nanomaterials.