Also, try this on the most south side of the settlement that you can, it's entirely possible the center of the settlement won't get you mk7s, but the far south side will! Then, once you've placed them, you can always move them where ever they are needed. Do this a couple of times and you should get a good idea of what you can get. Your best bet is to switch to a different turret and then back, and see what mk it is now. This is completely random though, you could be in a settlement that can get mk7s, and it'll give you a mk1. Since it shows -before- you place it, you can determine if you want to or not. There are the ceiling and wall-mounted bubble turrets that can be found in pre-War locations around.

This will tell you what mk the turret is going to be when you place it. There are three main types of turrets in the Commonwealth. But how do you tell? Before you place the turret, rotate it so that you can see the markings on the side.

However, if you are further south, it might be possible to get the mk7 turrets with a few tries. If you are sitting in Sanctuary, you can only get mk1 heavy machine gun turrets, which are far inferior to the missile launcher turrets. Built as a one-off experiment in multi-hull design, and jet turbine propulsion, the Bobcat features two automated Phalanx turrets, 22 ship-to-ship missiles, and eight floating proximity mines. Crew: 1 Helmsman, and 2 minelaying technicians. The turrets you are going to be using depends largely on your settlement. Engines: 2 sealed high velocity jet turbines.

Fallout 4 bubble turrets