I wouldn't be surprised if some type of DRM appeared on printers to prevent this
3D-printers are actually pretty easy to build even at home and if you build one of your own there wouldn't be any sort of a DRM.
The DRM could look for blueprint designs by hash, or certain "gun-like" items.
Impossible. There is no way for the printer or the software to know what the parts will be used for. There is no universal definition for "gun-like" as even a simple, straight tube would be "gun-like."