Just a quick disclaimer, this is not a complete guide, but it is a start.
Have you ever looked at your
40k tanks and decided they'd look better with a cold war vintage M60 style 50cal armed cupola turret for the commander?
Well if not, why the hell not?
If its because you can't be bothered modelling it, hadn't thought of it before or think the idea absurd; well fair enough. But here is a quick tutorial anyways. Just in case you were wondering if you could really fit another pair of turrets onto your baneblade. I mean, in case you wanted to make something pretty novel and cool.
Materials
Hobby saw of some description
Hobby knife of some description
Spacemarine cupola from the rhino etc.
Spacemarine vehicle upgrade targeter mounting
Milliputt or a form of modelling putty you are happy to use.
IG tank Heavy Stubber
some thin plastic rod
So here are my components the Rhino cupola is a bit chewed up as it was originally surplus to requirement and I hacked a large periscope off it to use as a sight for another conversion project. That perescope would be useful in this project and if you have one on your cupola hack the sucker off and glue it on the top later.
So immediately you want to find the front and centre of the rhino cupola and cut a chunk out to give a nice flat verticle surface to put the machinegun onto. Secondly cut the machinegun short. Then cut excess detail from the other components, they will be covered in putty later and you do not want random things poking out of it. Also take the targeting mount and cut it in half where I have done such here.
Glue the larger portion of the targeter mount will be the front part of the turret so glue it in place. But the thing should be taking shape.
Now it might look a little tight, and we have our little tank commander and a handy baneblade turret here to add to show the concept.
The hatch at the rear here is bluetacked in place.
Make some putty sausages and put them on the turret like this
Work and smooth the putty together. I did not build the putty up on the roof and did not work on the back of the turret till this set completely. Also make sure with your knife that you have a nice square hole to the front to later sculpt a canvas dust cover over that area. I left a big cut out so I can glue a kind of optic there, I'll see what I have in my bits box.
Make another sausage whilst the last one sets. Put this on the hatch, smooth out and such.
Once the hatch component is set glue a short section of plastic to the rear centre of it to act as the hinge unit.
So Mr tank commander in a test fit. Its a little tiny for a
40k tank hatch but ends up a respectable size in comparison to an actual guardsman.
Once both other components all set make another sausage and put it on the rear of the cupola, smooth this down and work the hatch into position. I feel kinda bad just saying 'make it work' 'make it fit' but that is what you gotta do.
You should have something essentially like this:
So here is one relatively finished earlier on an APC, because these can certainly be used as primary armmanets for something like a 30k Aurochs.

Obviously the turret is not detailed. I'll add an optic next to the machinegun, a perescope at the top and some plasticard vision ports.
I am not really finished this project and am currently lacking in key bits. Here are some references to help with finishing the things in any case.