Sentry Gun [28mm]

I couldn't get the [Sentry Gun](https://www.thingiverse.com/thing:1063180) by [bulldog2708](https://www.thingiverse.com/bulldog2708) to print reliably or nearly as well as their demo pic, so I reworked it to be more robust and come out more cleanly.

No rafts or supports are required. Print 4 of the legs.

There are four versions of the heavy machine guns:
* Regular, which has some minor overhangs in the barrel that interact with the thin walls to sometimes deform a little in printing.
* Fully supported, which doesn't have a fully hollow barrel and extends the bottom bar all the way out to the tip of the nozzle, so the barrel in theory doesn't look quite as cool, but in practice won't deform as much so might look better after all (and the insides can be fully drilled out if desired).
* Split, which print as two halves on end to be glued together, so supports aren't needed but the overhangs are maintained.
* Split v2, which moves the split point up just behind the barrel. Th