Actually, reading it up.
A compact carbon-nanotube based fuel cell is the best solution for the power source issue. It can effectively store 100 times more than Li-ion batteries so that means you can indeed jack up plenty of megajoules of charge into less than a kilogram.
So yeah, railgun rifles can be pretty much a reality for this RP. Again, if you think about some kind of superweapon which shoots through everything then you played too much videogames.
In effect you just gave yourself a gun which while performs somewhat better than firearms but its real advantages are elsewhere.
More ammo, almost arbitrarily adjustable projectile speeds (you need a tiny bullet accelerated to hypersonic velocities? You got it!), better rate of fire (basically only limited by the speed which the rail can slide forward then back) and more evenly distributed recoil (which would require to change the design of guns drastically). If the design is tried and proven like such a future setting then I suppose cost won't be that much in comparison (still many times more expensive than a simple gun) and it'll be reliable like any firearm (or perhaps even more).
So yeah, I am actually now thinking of giving railguns to my infantry.Well, probably not to all of them.