So what is wrong with a Stalker Boltgun as a sniper rifle? Why are you putting something as mundane as a sniper rifle that shoots mere bullets as having nearly the same effect as a rocket-gyro-propelled grenade-missile sniper rifle or some other, similarly wtfinsane heavy-duty tech-heavy weaponry? Keep in mind the setting - bullets tend to have this "richochet" thing when they're aimed at Deathwatch Astartes. It literally takes a hail of bullets to even POSSIBLY damage a Space Marine. What can hurt a space marine? Super-tech heavy (melta, plasma), RPG shooter (bolter), and monstrously nasty death in melee (Powerfists and the like).
Have you modified your intended Sniper Rifle stats in any way during this discussion? If so, please share - we'll help you balance it into the system. (Some of us might be snarky, though).