So I'm working on a Highborn Grenadier Regiment to act as a rival for my player's regiment of Last Chancers, and I noticed that the entry for Grenadiers doesn't actually give them a main weapon. They have auxiliary grenade launchers, light carapace armor and other gear, but they still technically have a laspistol and two clips as their main weapon.

I'm giving them a lasgun and four clips like most other regiments, I assume this is correct?

I do not believe so. In this case, the soldiers of the grenadier regiment would acquire any additional weapons from their specialty and/or Logistics. Thus not every member of a squad in this regiment would have a lasgun, but they would all have laspistols, grenade launchers, and light carapace.

Upgrade the Laspistol using the 30 points you have for additional kit.

I expect it is an oversight. It makes no sense (rules wise, not Munitorum wise) to give them as standard equipment an auxiliary grenade launcher but not a basic weapon for it to be attached too. It is a weapon upgrade for one thing and therefore has to be an upgrade to something.

Personally I would be leaning to give them Lascarbines rather than Lasguns, as it is closer to a pistol and seems (IMHO) to fit the Grenadiers image better.

You'd still have your Favoured weapons, so your Weapon Specialists and Heavy Gunners get the Favoured weapons you choose, while all the others just get the las pistol.

Maybe the auxiliary grenade launcher works like the M203 or M320, which can be equipped with stocks to be used standalone or in an underslung configuration.

Many of the classes, in addition to the regiments, bring their own gun, whether a lasgun, a Heavy, or an Assault weapon, so they might've thought that the class would cover it, and the others (Medics and Operators, mostly) would have pistols, and do their supporting job. beyond that, maybe it's an oversight, but I figure that most types get a gun that could have the aux launcher, if they WANTED it (that would be a silly heavy bolter), while Commissars, TP's, Psykers, Storm Troopers, Ogryn, etc, all do their own thing, and ignore the upgrade. It doesn't seem as weird to me.

yeah that is kinda weird.....though if you give them bolters......