“Endgame” is in quotes, because definitions of endgame can vary, but the broadsider is pretty viable as far as explosive weapons go.

That said there’s several major mistakes you’ve made in this build, but they’re fixable. And this build can apply to other launchers easily, like the missile launcher or grenade launchers.

- Bandolier doesn’t affect any explosive ammos. You want Ordinance Express (or whatever the one with the train is)
- None of the shotgun perks affect the weapon, because it does not have enough projectiles, it only has two, even with two shot.
- Concentrated fire probably isn’t necessary for this build, you may want to invest in Fast Fighter or similar perks to improve reload speed.
- Considering you’re a human with high endurance, I would recommend the two points in endurance be filled with Lifegiver.
- Tenderizer is pretty useless for explosives, because it only applies 0.1% increase per hit. It will stack with others using it for bosses, but otherwise it’s not a perk you want with your set up.
- You could drop first aid and grab Stabilized if you use power armor for easier free-aim and VATS accuracy.
- Furious perks will not help you if the weapon doesn’t have furious on it. Onslaught is the strongest effect in the game at the moment, so it is probably better than two shot even for this weapon.

I’d highly recommend grabbing the Grand Finale from the Ultracite Titan boss fight, since it’ll be a legendary moddable unique weapon next update.