In Dark Heresy one of the characters in my group has grown very fond of her Long las. Combined with high BS, Talents and Hotshot-shells it is doing pretty bad-ass damage. For marine sniping purposes the Sniper Rifle and the Stalker pattern both do their stuff. One is for ultra-stealthy soft-target elimination, other for more general purpose dedicated marksmanship job. If you need to positively kill every Tyranid in the room then you are better off with heavy weapons like Heavy Bolter or Missile Launcher anyway.
One thing I would change, though, is that Sniper Rifle is Exotic Weapon. I mean 90% of existing chapters have their young scouts use that weapon so how in earth doesn't a basic marine have the skills to use it? If you are building a starting character and want him to be a sniper of sorts, I would suggest Tactical Marine with standard Godwyn Pattern bolter. Give him some longer range bolts (Kraken rounds, if I remember correct) and targeting scope for the bolter and he will do his job just fine. Sniper Rifle is a special tool, for special jobs, not something a squad dedicated marksman would carry to battle. God knows US army dedicated marksmen sometimes do their job with simple M4 + scope (although they really shouldn't).
BTW, unlike Hollywood says "sniper rifle" is not a synonym of .50 BMG and "snipers" do much more than just extremely long-range shooting... In my youth in army I did qualify for sniper and later helped in training some more. The weapons used by the snipers varied, according to job at hand, from silenced .22 LR to 14,5 mm heavy machinegun.