When I think of a well-performing closed bolt marker, I think of Excaliburs. They're faster than Shocker Sports and pVI Shockers, more efficient than Shockers, and much more reliable and solid than the typical Autococker. The Excal design is solid and proven. Even though these markers are no longer produced today, their resale isn't horrid and they still have some value, especially with Big Game and scenario players.
I don't have an extreme amount of information about them, but I have taken the time to amass a few bits of tech info here and there.
Design & layout
Solenoids (MAC or Humphrey)
2003 Excalibur manual in pdf form