If you work out the maths and physics behind the muzzle velocity you arrive at the leathality:
http://miscarriageofjustice.co/index.php?topic=11191.msg570652#msg570652
And if you want a practical example look no further than the case of David Bain where 5 family members died from gunshot wounds inflicted from a. 22 rifle where 8 cartridges were used.
The maths in context are way more useful.
In this case it took several rounds to finally dispatch his mother and father.
But if a more suitable weapon was at his disposal, would it be used? Of course it would, but that wasn't possible in this instance due to the necessity to make the pieces of the puzzle fit.
Let's look at it from another angle - how else could Bamber do it any retain any hope of getting away with it?
Hitman? Finding one is difficult, finding one that will kill kids even more so, plus where's the motive?
Robbery gone wrong? Nope. Why kill everyone? Why not just leg it?
Fire? Non starter as I've recently learned - he considered it, but then dismissed it as anything of value would be destroyed and the insurance (that he subsequently reviewed) didn't cover it in his opinion.
As bizarre as it sounds, the only viable mode was the one he cobbled together - the Sheila Went Bonkers angle.