This is a fascinating experimental Remington Keene rifle, build in an attempt to alleviate concerns about magazine tube detonation. With centerfire ammunition in a linear tube magazine, there was an ever-present (if very slim) risk of a bullet detonating the primer ahead that it was resting against. Remington’s experiment here was to add a second magazine of donut-like spacers to separate bullets from primers in the magazine. These spacers were manually inserted between cartridges when loading the rifle, and they were automatically returned home when cycling the action, so as to not be disposable items. The system was clearly too complex to be practical, and the move to box magazines removed the issue entirely before long.

Interestingly, this prototype was also designed to cock automatically when the bolt was closed, where the standard production Remington-Keene rifles head to be manually cocked.