...and how to fix it.
Or at least I think so.
I took some demos of myself playing combat, and then watched the parts where I was shooting at skulks with a shotgun at 1/8 speed. And I noticed that sometimes I'd shoot at a skulk that was pretty close, and I'd be right on with my aim, but the skulk would live, and he'd be surrounded by sparks, although there'd be a little bit of green blood. Allow me to reiterate that this isn't misplaced ego or whatever - I was watching at one eighth speed
and my reticle was right on
the skulk (I use the old 1.04 crosshair for every gun but the gl).
So then I went and started a local game, turned on sv_cheats, shot the walls from a medium range a few times, and looked at the bullet holes. Sure enough, the spread is random, and sometimes there are holes in it, or a side is weighted more heavily with pellets than another, or whatever. Skulks are small enough that they can fit inside these little irregularities. Additionally, I'd sometimes get kills that should have been woundings because the load happened to congregate where the skulk was.
So, how to fix it? Simple, make the ns shotty work the way real shotties work. Give it a consistent, predictable, and even spread. Easy!