Muzzle Thread Protector

This screw-on sleeve is designed to protect the muzzle threads of certain firearm barrels when no muzzle device is installed. Its circumference is ribbed for grip so that it can be screwed on and off by hand. Supported thread specs are:
- 1/2"-28, commonly used for .223 Remington and similar chamberings
- 9/16"-24, commonly used for .40S&W, and sometimes for 6.5 Grendel
- I fitted this to a [Grendel Incursion barrel](https://www.shopalexanderarms.com/DIY_Parts_Components-6_5_Grendel_16_Incursion_Barrel.html) by Alexander Arms and found it to thread on perfectly.
- 5/8"-24, commonly used for 6.5 Grendel/Creedmoor and for various .30-caliber chamberings

Additional thread specs to be supported in the future:
- M15x1, commonly used for 7.62x51 mm
- M14x1 LH, commonly used for 7.62x39 mm

It's not yet clear to me how this thread protector will handle the heat of the barrel under normal use, so this may very well be a pointless endeavor aside from the learning experience regarding thread specs and maximum temperatures.

The threads were generated with Dan Kirshner's [Threads library](http://dkprojects.net/openscad-threads/) in OpenSCAD, using thread diameters oversized by 0.5 mm.