I had two ideas today: the first is to have some sort of
gyrojet weapon, based on a quick check on wikipedia
it would do its best damage at mid range, the gun would be light and probably easy to make, while the bullets possibly could be heavier and difficult to manufacture.
The second, for if we end up with aquatic missions is loosely related, I found out that there are guns designed for
underwater use, they generally shoot flechettes. Makes me wonder why X-Com went with spear guns and dart guns, as these appear to a bit superior.
Edit
Might have figured out the speargun thing, effective range apparently goes down the deeper you go with the underwater firearms.