There are two flag fields in an action actor. The one you're looking for is called Actor - Action Flags, and is at the very top when sorting by names of fields. Also, make sure the Art - Missile field is set to the right missile actor. If this field is not set, everything will work except the missiles will come from the base of the unit like you describe.
I decided to try make a missile tank with a combination of the Siege Tank body and the Missile Turret.
I've also attempted the under hull viking cannon, but that doesn't seem very doable since the turret rotation also flips when you flip the gun up side down.
To save you some time. Don't bother with trying to use the Turrets without the body. At least not using this detach method. As you've found out yourself, they will either remain in one spot, or they will follow the unit as they're supposed to. In addition, using triggers to move the turrets periodically doesn't look good, so you shouldn't waste your time doing that either. The only use for detaching the turret is if you want to use the unit's body.
You can break a turret from it's body actually. It's a workaround, but could be useful to you. Basically, if you try to set the position of an attached turret, the turret gets detached, and you can move the tank body around without the turret following.
To set an attachment's position, use a trigger to send an actor message of type Attachment Set Position.
Edit: Here's an example map. Note that I've disabled the turret actors for these units. If a unit has a turret actor, you won't be able to modify the turret's position
@Nahotnoj: Go
There are two flag fields in an action actor. The one you're looking for is called Actor - Action Flags, and is at the very top when sorting by names of fields. Also, make sure the Art - Missile field is set to the right missile actor. If this field is not set, everything will work except the missiles will come from the base of the unit like you describe.
@D1000: Go
By using an invisible Phoenix to link the immortal base and the templar, you can have the templar rotate when attacking.
@Lilianna: Go
Very clever. The turret's body rotates as well, but it's not very visible when it's under the ship, so it worked quite well.
I ended up using an invisible phoenix model for the link between ship and turret due to it having Origin and TurretZ in the same position.
I decided to try make a missile tank with a combination of the Siege Tank body and the Missile Turret.
I've also attempted the under hull viking cannon, but that doesn't seem very doable since the turret rotation also flips when you flip the gun up side down.
@Lilianna: Go
To save you some time. Don't bother with trying to use the Turrets without the body. At least not using this detach method. As you've found out yourself, they will either remain in one spot, or they will follow the unit as they're supposed to. In addition, using triggers to move the turrets periodically doesn't look good, so you shouldn't waste your time doing that either. The only use for detaching the turret is if you want to use the unit's body.
@Lilianna: Go
That's correct. Hope it'll be useful to you
You can break a turret from it's body actually. It's a workaround, but could be useful to you. Basically, if you try to set the position of an attached turret, the turret gets detached, and you can move the tank body around without the turret following.
To set an attachment's position, use a trigger to send an actor message of type Attachment Set Position.
Edit: Here's an example map. Note that I've disabled the turret actors for these units. If a unit has a turret actor, you won't be able to modify the turret's position