The exact chemical composition of gun powder is going to be variable over time, but consistent within a given batch. Which tells you the chemical composition is going to be consistent for a given lot or pallet of ammo. We can assume the FBI is procuring ammo by the lot/pallet and there is a crap ton of paperwork involved. Any powder recovered from Seth Rich, presumably shot at point blank renge, might perfectly match the ammo currently in FBI inventory, sitting on a shelf at the FBI. But, that little bit of investigation would probably end your career at the FBI if not get you suicided.