Remington eXperimental Pistol number 100 “Fireball”
Manufactured by Remington Arms c.1963-1998 - serial number
.221 Remington Fireball single shot, bullpup bolt action, nylon target grip.
The Model XP-100 was made specifically for long-range pistol shooting, a thing it indeed excels at by using the fastest handgun cartridge commercially available. Which also happens to be a rifle cartridge.
Space gun !