Miniguns cannot be repaired with grenade launchers, even with Jury Rigging.

Prior to patch 1.4.0.525, the grenade launcher could be repaired with miniguns.

The grenade launcher is based on the China Lake model grenade launcher from the late 1960s.

Characters with high agility and/or Rapid Reload may prefer the lighter and simpler grenade rifle, as with quick reloads it can fire faster than the grenade launcher.

There is one carried by one of the caravan guards at the beginning of Honest Hearts.

They can be found on some of the White Legs in the add-on Honest Hearts.

The munitions trader at Nellis Air Force Base.

You can find quite a bit of ammunition for it inside Nellis Array near Nellis Air Force Base where the Boomers reside.

Two of them can be found in the Armory of Vault 34, along with some 40mm grenade ammunition.

Can be found at Nellis Air Force Base and on most Boomers, notably during the Ant Misbehavin' quest.

Grenade launchers can also be purchased from Contreras at the Camp McCarran supply shack or from Blake at the Crimson Caravan Company.

One can be bought from the NCR arms merchant at the 188 Trading post.

While the feed system makes the launcher significantly heavier than a standard grenade rifle, the launcher's body makes an excellent platform for experimental energy weapon conversions.

Leaf iron sights similar to the grenade rifle are mounted on it. A pump-action weapon with a tubular magazine holding four 40mm grenades, the grenade launcher is an excellent tool for delivering explosives over long distances.

Fallout 3 grenade launcher