When I think about the weapons for whom accuracy is to be a reasonable replacement for sheer quantity of bullets, I expect a well-placed shot to do similarly and correlatingly well in the Inflicting Damage department. Where a hail of bullets might stop the armored truck, I'd expect a precise shot could do the same or better by striking weak points in armor or destroying/hampering necessary systems and functions.
I think what I'm saying is that weapons that do very precisely what you want them to do, in the hands of the real expert shots (like your aforementioned assassin), really aught to do some truly stupendous things. There's not much that should stop a well-placed, defenses-defying, no-warning solid slug of metal (or destructive energy) to the face, barring something inhuman like a Space Marine or a Daemon.
I do agree that Semi-auto and Auto aught to be separated more, in terms of giving each firing mode a unique touch.
Not only should they be given unique touches, but perhaps Pistols could be given some of their big brother Basic's ability. Uncle Heavy could perhaps be the next step out: Always +1d10 damage for every 2 degrees of success, but Pistols are limited to +1d10, Basic to +2d10, and Heavy to +3d10.
When the Spray-and-prayers are busting out Autocannons as dakka incarnate, a dedicated sureshot could step up with a Heavy Basic weapon with the Accurate Quality and the sheer destructive power of a heavy armnr-piercing round, and put a much larger chunk of penetrating metal hurtling at a much higher velocity with a much larger force, and even the Speeeeeeece Mahreeeeeeeeeens aren't getting up from the round you put through their noggin (or heart, or neck). And that power supply or gas tank or relay station for that vehicle? Friction-irradiating precision means hot death just went straight through it.
Of course, all weapons pale in comparison to The Radical Handbook's Xenarch Death-Arc, capable (despite its inability to benefit from Aim) of both a spray of up to 6 lasgun-strength shots and, simultaneously, the ability to only treat armor and toughness once against multiple hits! Yes, ladies and gentlemen, that means that Accurate's paltry +2d10 damage is actually pretty **** lightweight against xenotech, when a good/lucky shot on full auto could end up at six times a lasgun's damage for a single target, deducting armor and toughness only once?
(Did I mention it also has the Shocking quality and decent range? Don't even get me started on Shuriken technology...)