Science fiction is about hypothetical advancements in technology and strange discoveries. Consider that
incendiary ammunition and
high-explosive incendiary rounds exist, and that there have been folks working on creating a sort of "
shock round" or electric bullets for many years. Not to mention what technology may come from the inherently unpredictable interaction with alien planets and species. One has to acknowledge that in the context of science fiction (theoretical advanced technology and alien tech) elemental ammo is not one of the more far fetched ideas.
The basic assumption of Sci Fi is that we have advanced beyond what we have now. The farther we go forward the more advancements we should expect. What we know now acts as a certain tether, a grounding ideal so to speak, but we have to expect that what is considered outlandish today will be commonplace in the future. History has taught us that.
Whether one likes the idea of elemental rounds or not is a separate issue of course.