Bready said:
It depends on what you're trying to hurt them with, but then, that is the point. With lasguns and autoguns alone, bringing down an Ork (TB8, and lets assume 2 points of primitive armour) is very difficult and takes both a lot of time and a lot of shots. But it isn't impossible. It's soaking 9 points of damage per shot... but a Lasgun (or equivalent 1d10+3 damage weapon) can deal more than that - you just need to roll a 7 or higher to deal 1 or more wounds.
IMO, that's a good thing - if it's sufficiently tough to justify giving it Unnatural Toughness, then it should be extremely resilient... and it just makes Orks scarier, as they can weather hails of bullets that would reduce a human being to a pile of meat... often without noticing. Orkflesh is extraordinarily tough, the Orks themselves largely unconcerned with pain or physical injury... doesn't matter that bullets are digging into their flesh and lasburns are pock-marking their bodies, they just keep on fighting.
And then the Chimera's multilaser unleashes its own brand of hell. Each hit is still dealing 3d10-5 damage after reductions for Toughness Bonus, and on average, it'll take two of those to drop an Ork and start dealing Critical Damage (or kill it, if you're using sudden-death criticals).