Found a workaround to run the Bullet Mk1
in Orbiter 2016. If you copy a 'spacecraft3.dll', which is part of the spacecraft4
package directly into the Modules folder, the Bullet works (minus the main drive exhaust).
The Bullet brings along its own spacecraft3.dll in the Modules/Vessels folder, but this seems outdated.
By copying the newer spacecraft3.dll directly into the modules folder, this has priority and now the addon works.
(The dll-file is attached for convenience; the full package is at DanSteph's homepage
The Bullet has an insane ISP and high thrust. You can basically point to the target planet in planetarium mode [F9] and accelerate.
Don't forget to flip around in time to decelerate!