a point: a railgun's round is going to touch the rails.
thats how they work. the alternative, a
coilgun, will have a normal barrel which the round fits into, including rifling.
a railgun also does not need a metal slug. infact it works best as a non-metal bit, or as a core of a heavy metal in a non-metal jacket. all it needs is a conductive bit at the rear to bridge the gap between rails. a coilgun requires a metal slug because it uses magnetic feilds to work. (railguns just exploit a condition caused by electrical current)
generally, rifts 'railguns' behave more like coilguns.
i would point out that you can increase the damage of a rail or coilgun by modifying it's muzzle velocity..since it's a kinetic weapon, the faster it's projectile goes, the more damage it does. however this increases the recoil, certainly beyond the built in recoil management systems ability to counter. i'd say, increases the P.S. requirement to fire "offhand" (as a rifle, not a tripod mount) by 50%, and even as a tripod or fixed mount i'd say you'd be looking at a P.S. requirement to keep it steady.
personally, i prefer to assume the powerpacks are micro-fission plants using Uranium or Thorium...but use what you prefer. canon is still (annoyingly) slpit over the issue. (pre-RUE books assume fission/Atomic, as shown by their fluff. post RUE you have more fusion references...)