Actually, machine guns do in fact load above the barrel. Only slightly though. The round is pushed forward by the bolt and then pushed down in to chamber. This is done in this manner because the spent casings are ejected from the bottom of the weapon and the links that hold the rounds together in a belt, also known as a disintegrating belt, are ejected to the side. As far as hand cannons are concerned, if you look at the ammunition cylinder that is used in the weapon it appears to have a pin in the middle of it. This could be a firing pin extension. This similar to the way Barreta makes or used to make their pistols. They would spilt the firing pin into two parts and one part would actually be incorporated in to the safety mechanism. As far as hand cannons go in Destiny. I imagine that what happens is that a round is dropped into a center chamber within the ammunition cylinder where it lines up with the firing pinm once the round is fired it returns to its original position. This would actually be an ideal design as it would shift the recoil from the top of the weapon to the bottom of the weapon. The rocket launchers are a bit perplexing but it's possible the round are loaded into a single cartridge one behind the other. This would be similar to the way Metalstorm weapons are, where they use a stick of ammunition loaded into the barrel and the weapon. If you want to know anything else feel free to ask. I worked on weapons for a little over year so I hve a pretty decent handle on some things.