So if you order a colonist to man a turret, you can order another colonist to stand one the same tile as them.

If you undraft and redraft the turret manning colonist, you can then order a third colonist to man the turret, and repeat this process until the entire colony is standing on a single tile.

I had 10 people with shotguns clustered up behind the manned turret (which provides 85% cover).

And yes, they one-shot every signle raider.

Edit: This bug is in A17.