Question about the (super)shotgun and/or its pellets...

You can even shoot through enemies with the freeze command, which should stop tics entirely. Try it, use the freeze command in a ZDoom based port and use a hitscan weapon like the SSG.

Arctangent said:

However, they become unshootable as soon as they die, likely before their state is even changed to their death state.

Although this "unshootability" can be observed in typical scenarios, in fact, as soon as monsters die, they only lose their ability to take damage, ability to be autoaimed at, and their height and width is reduced to 1/4 of its default value, but they don't lose their shootable-ness immediately. If you manage to shoot them low-enough at their feet (and quickly enough), projectiles and even hitscans (but I'm not 100% certain about the hitscans) would collide with the dead body, until it becomes non-solid a couple frames later during the death animation. I'm pretty sure that ZDoom changes this behavior, though.

Well, here's the original thread where this was mentioned. Printz started it, then Gez did a more in-depth analysis, but TBQH I never verified those claims, either by gameplay or by code analysis.