Universal Ammunition: Not quite universal, but so far as regular guns go (see Bottomless Magazines for the description), there are very few different types of standard magazine cartridges (metal for slicing up), powercells (which are just the fast-discharge versions of regular battery equivalents), and heat sinks which cover virtually everything on the market.  And even where slugguns are concerned, there aren’t all that many different bore sizes beyond the two main ones, and the rest of the compatibility issues can often be handled by software patch.

Conventionally averted where the exotica are concerned, though.