Weapons don't' have a Critical Chance, they have a critical chance multiplier. Ratslayer's is 5x. So, if you character's CC is at least 20% then yes, you'd always score a critical hit. But Ratslayer is a Varmit Rifle, not exactly what you'd chose to hunt mutants or cazadore's with, even with a 100% CC because the critical damage is so low. They game is well balanced in this regard. Don't underestimate the Ratslayer. For a stealth character at range with the night vision scope and the silencer, maxxed guns, and always critting, it can be a very potent weapon. Even if it takes more than one shot it doesn't matter because they never see you anyway.