True, the automatic grenade launchers IRL are not moble, they are "man portable" which means that people can carry it, then set it up and fire it.
And how is automatic grenade fire hard to adjucate? Roll once for the burst, then deviate each grenade based upon that single roll. Recoil modifiers apply, since you can't stick gas vent on the thing. And since it is a heavy weapon, uncompensated recoil counts double. The best you can hope for is a gyromount. a tripod, or a vehicle mount (which is where you find them most often, anyway). You wanna spray those around, anyway, for overlapping explosive arcs.
One could always say the overlapping explosive radii on a target simply add +1 to the base DV of the attack (the same as a normal burst) or you could boost the base DV of a single grenade like a burst and increase the explosive radius that one (to simplify things). So a short burst creates a single grenade attack with an increased DV of 2 for a narrow burst, or a reduced defense of -2 for a wide burst (you are catching them in the explosion). That's if you want simplicity. I'd resolve grenade damage separately, personally.