A rail rifle is a bit weird to get hold of.
And to keep ammo supply.

As for alternatives.
Hellgun sniper. Either just hot-shot pack or a proper hellgun sniper. It packs a punch.

The exitus is probably the best one.... but then again it's the exitus. So no.

Hmm, if you like being illegal.
Angelus bolt carbine. It's a rifle, one shot per clip. Goes well with a scope.
It fires stolen astartes bolt shells. Meaning in dark heresy it is a mean weapon indeed :P

Otherwise, having a regular slug rifle. One can load it with some interesting ammo.
Man-stopper, incindiary and explosive are just a few examples. Or toxic, toxic is fun.

Or for maximum lulz.
Get an autocannon, only load a single slug. Add scope. Eyyyyyy!!!

Or a lascannon :P

Looking on the rail rifle, i saw this.
"[i]While a Rail Rifle is conceptually simple, manufacturing and fielding one presents several engineering challenges, requiring highly conductive materials with strong resistance to thermal expansion, efficient cooling systems, very precise machining processes, and methods of quickly servicing and re-machining the rails in the field to keep their interior surfaces in pristine condition. Even minor wear can have a significantly deleterious effect upon the weapon's performance.[/i]"

Seems like too much of a bother to me.

Or you could come up with a local variant of hunting rifle.
Perhaps one planet had a big problem with roaming ogryns/big beasties. And developed a high calibre high accuracy weapon for sniping those sods.

Or a bolter. They are reasonably accurate, unless you fire full auto.