I don't know what game type you was playing but in competitive modes people don't take your downs unless you are occupied with an already alive enemy to stop the downed enemy getting back up.

As for the Gnasher I don't know, I find its very particular about distance if you are literally touching the other player then it is always a one shot kill anywhere. You just have to practice a bit more and figure out that sweet spot (distance) between instant headshot or two body shots.

The gnasher is fine, however shotgun warfare is inherently inconsistent. Playing gears there will be some times where you shoot people point blank and you get cheesed its how its always been. Im a little jealous of the person above me because my experience with downs is in line with OP almost everybody takes everybody elses downs right now. Thats just because of the heavy amount of new players which will dissipate, plus if youre playing competitive you can have my kills the W is all that matters it actually improves your gameplay in competition if you dont care about such things.