[EDIT: Fixed. Thanks!]
Can you elaborate? Making both the Mass Driver HWP and its ammo have the same prerequisites, which is "HWP Mass Driver". This research depends on "MASS DRIVER CANNON" and "Heavy Weapon Platforms".
Alright, i'll explain
Mass driver tanks and its ammo can be manufactured by XCOM if you just research UFO powersystems and Mass Driver Cannons (I got mass driver weapon from Osirion loot box).
But you cannot make your own mass driver weapons for infantry. You can make one for vehicles... but you
cannot make handheld versions.
To be able to make a mass driver weapon for a HUMAN you need:
-
ELERIUM battery <---- this one is locked in RNG rullete of Engineer/Leader integoriation
- Mass Driver weapons
To be able to make a mass driver for a TANK, you need this
- UFO power systems <--- this one is plausible in early game
- Mass Driver weapons
So XCOM can make tanks, but not rifles and pistols. Like there is some logic in this if you want to leave it as it is because its much more plausible that XCOM can make these railguns on vehicles because they can be equiped by heavy batteries to power it and the tank has the strenght to move around with them, but handheld versions need alien grade batteries because otherwise the gun would be unusable by humans from its shere weight to be able to even power such thing.