Not entirely sure it's possible, but wondered if the Assault rifle could be modded with a 4th additional ammo selection for fragmentation grenades that could be fired or (launched) from the rifle like similar most assault rifles with a grenade launcher attachment under the barrel. Essentially providing a bit of extra explosive firepower to the last high end weapon for standard weapons without needing carry the Brick Grenade Launcher (though limited to only frags) but not sure if it would work properly as the rifle's setup would treat the grenade rounds similar to bullets and either fire it rapidly with no delay between shots.

Alternatively it could it's own custom weapon that replaces the normal assault rifle. The firing mode being standard auto with the other being the grenade launcher firing mode which switches the ammo out to the grenade launcher ammo set and has a delayed firing rate similar to the Laser Pistol's overcharge setting.

The other possible weapon was an alternative high tier explosive heavy weapon, essentially a upgrade of the TC-11 Brick grenade launcher but a rocket launcher variation (or RPG), possibly using the same firing method and projectile the Blast Turret uses, delivering longer range with a heavier punch, larger blast and radius and utilizing the specialized variations of the grenades. The firing modes could be the standard aim and shoot mode. While the other could be either a heat seeking variation from the Annelid Launcher, or a sort of remote guided mode which you can briefly steer the rocket to a target. The drawbacks could be a reduced movement speed while equipped, a high strength requirement and while each shot does more damage it consumes 2 grenades per shot.

should be doable.

require the tinker trait, must consume a grenade launcher to upgrade the rifle, do the minigame and spend a lot of nanites to combine and require a high modify skill to pass the check, nerf grenade damage and speed, good to go.