This was a problem in pre-Black Crusade game lines, as Full Auto gave bonuses to hit and extra hits per turn. With the revision that gave it a penalty, and a much-needed upgrade to the Accurate special quality (granting it bonus damage), things became better for Single Shot.
This is no longer the case. As Wounds are no longer a hit point system, and mooks don't simply die at 0, damage is now tracked via progression on a lengthy chart of effects. Hitting a target who has already been hit grants a +5 on this chart.
As such, Full-Auto will be popular. Assuming you hit twice, the first shot will grant the second one a +5 bonus to the chart result. Further hit will most likely kill the target in one burst.
Conversely, a Sniper Rifle is incapable of killing someone in a single hit. Let's say our Sniper Takes aim and hits some poor sod right between the eyes. Even if the target is a normal human being not wearing armor, this hit will not kill him. Unless the hit scores Righteous Fury, the best result the sniper can hope for is a 20 on the Impact - Head table, which is not only non-lethal, but it doesn't even inflict Blood Loss! There is no way to kill someone by shooting them once in the head!
This is absurd!
With the rules-as-written, an elderly Administratum Clerk shot in the skull with a sniper rifle will be stunned for ten seconds, then shake his head and walk it off.
I hope you can see this issue with this.