I've been looking into this. I can tell you for sure to hold on to your marks for now, infusing gets it to 170 nothing further. I got a second strangers rifle, maxed it, then dismantled it in the hopes of getting something from it and no, nothing. 50 marks wasted. But you don't have too, gotta try doing the mission in a harder difficulty maybe, or ranking up gunsmith further. A quest needs to be obtained for it to be ascended I'm pretty sure of that, I don't think it's a drop, seems too much like a story plot point for a random drop.