On page 111 it is "The Automatics skill governs the use of
personal firearms larger than a pistol,
capable of autofire. . ." (Italics mine).
So we've got two problems trying to use a grenade launcher with this skill. First of all, grenade launchers are kinda
beefy to class with personal firearms. Although the NRA might agree with that designation, come to think.
Even if you do consider a grenade launcher a personal firearm (and as opposed to what, an impersonal firearm?) there's still the small matter that grenade launchers aren't capable of autofire. Hence the
automatics skill seems somehow inappropriate.
While it's true that the heavy weapons skill refers to items "larger than an assault rifle" and the underbarrel grenade launcher is by definition smaller, that by no means puts it in the realm of automatics. One could be a dick and require that a character take the Exotic Ranged Weapon: Unbarrel Grenade Launchers skill. One could make the same argument for a LAW rocket which is typically lighter than an assault rifle and (when closed) may be smaller!
Finally, "The Pistols skill governs the use of all types of hand-held firearms. . . ." Since a sniper rifle is hand held (how else do you hold it?) it is clearly governed by the pistols skill.
Fortunately, or unfortunately, Shadowrun 4th Edition wasn't written by lawyers. After having played a few games, I'm convinced it was written by gamers which is a damn good thing.