Anecdotal only, but the only times I've heard of a fourty mike mike actually killing someone by (unintentionally) hitting a target cause of death has been head trauma.
A 40mm grenade is propelled out of the barrel quickly as indicated by the above muzzle velocities, but keep in mind that it also loses velocity, and thus kinetic energy, significantly faster than even a subsonic bullet because of the mass of the projectile itself. Also keep in mind that unlike a bullet shaped to penetrate and with enough energy to actually do so, while a modern day 40mm grenade has a much larger surface area and so would need an equally significant velocity to penetrate a soft target; gunshot victims die from shock trauma or exsanguination more often than not, and a 40mm hitting you in the chest (especially if you're wearing armor) is therefore more likely to inflict Stun damage in my opinion. The exception would be headshots, of course, as a knock of that kind to the noggin would probably put you in the grave from blunt force trauma to the head.
If you ask me, getting hit by the Shadowrun microgrenade should only be lethal in the absolute worst case situation, meaning critical glitch; it's by definition an unlucky course of events. I'd liken the projectile more to a bean bag round than a slug, and give it a lower damage code but some special rules for what happens on a glitch or critical glitch, personally. There is such a thing as too much realism, and I'd draw the line at microgrenades inflicting shotgun-level damage.
So would a char get a defense test against getting hit by the projectile itself?
That's how I handled it the one time it came up; the player rolled his attack as normal and got enough hits to land his grenade on the target without scatter, and then the NPC rolled defense against the attack. He failed, took 6 stun modified by damage resistance, then had to resist the explosion of the proximity trigger grenade which blew up when the grenade hit.
