Good old SETh...
You can also do this with other entities that display a model. In the case they don't have a 'body' key, you can add one manually. I've had this working correctly with monster_human_grunts, cyclers, and so on.
In this way you can choose exactly how your grunts look
and define their weapon, instead of getting a random grunt each time. Technically, it should be possible to get grunts without weapons...