Folks saying greater than 380 are lying. The base rating is simply all the rolls added together, nothing more. Sometimes they won't add up to the rating (aka be 1% off), but I imagine that's because the ratings are like Vermintide 2 and have decimal values we can't see.

So theoretically the max is 400, and if we get red/legendary weapons beyond that then it'll be 500 (aka all 5 bars filled up to 100%).

Getting the highest rating is typically the best, but I'd still use other weapons that had good rolls on their important stats (i.e. ammo/damage).

Heck, getting a weapon with a bad stat in something like, say, stagger might actually help. Less stagger means easier to shoot multiple times.