I've been making a number of purchases lately to make some of my new 30k army look unique and pop a bit more on the table top. One of the ways that I'm doing this for my "regular" space marines is to change the bolter magazine. Anvil Industry (http://www.anvilindustry.co.uk) sells just these parts to help make this idea a solid reality.
The image pictures two sprues of 10 drum magazines each, each displaying opposite sides to the other. Each of these magazines will fit on to the side of a regular bolter (or even bolt pistol) to create a weapon that looks like it has heaps of bullets in its drum ready to expend. This is in keeping with the background material for the Horus Heresy, where it notes that latter Heresy developments included large drum capacities to ensure that marines didn't run out of bolt shells in the middle of protracted planetary assaults - particularly in the wake of the Isstvan incidents.
The drums themselves are resin, and three-quarters of a cylinder in shape. Bolters that they attach to will require a little bit of work, which I'll detail in a future post. Regardless, these are a terrific little purchase as they certainly help regular marines stand out from the crowd. I'll personally be using them on a gun-kata legion troops style squad.