So, I've stumbled on a pretty effective way to handle manhunter packs in my colony.

My colony's dug into the side of a mountain, with a naturally walled-in clearing separating it from the rest of the map. I built a small 5x5 foyer at the entrance, mostly as a way to airlock the colony and have some extra barriers in case a raid makes it past my defenses. It's also a good staging ground for handling manhunters as I discovered... especially if you have a pet following one of your colonists.

Since animals wander around drafted colonists they're assigned to, they occasionally open doors by walking through them, just for a moment - it turns out that in this brief window colonists can get off a couple shots through the open door, and it's also brief enough that manhunter animals don't react and start bashing down the door (or the pet) in retaliation. Easy pickings, and fresh meat for everyone!

Two LMGs, an assault rifle and a chain shotgun make very quick work of anything lingering outside - though I'll probably upgrade to charge rifles when I have the supplies.