Well, a sniperbuild would require you to excel at timing, aim etc. With only 30 shots/minute, a missed shot could mean trouble, and you might wanna make sure you got escape up before you engage a pack. The problem is you deal less dmg then a automatic rifle-based marksman vs packs. A sniperrifle dealing 300 dmg with 30 shots per minute compared to an automatic rifle dealing 30 dmg with 300 shots per minute, 10 zombies will still take you 10 hits (20 seconds) to kill, since they have low hp, while the automatic one will take 2, but still finish the job in a lot less time due to that. If you get a convulsor (sub only sniper rifle with large splash damage) it might work out better tho.
Then again, it does take a certain ammount of awareness and caution to survive with a BM.