These devices were constructed using a Krag action.

In Oct. 2017, 'Marksc' posted on the KCA Forum about a Krag that he had with no serial number!

With discussion and pictures, it became apparent his Krag action also lacked certain machining steps. Another forum member connected it to the sub-caliber devices.

William Brophy, "The Krag Rifle", pages 206-207, shows two types of devices:

1. One was a fixture, made circa WW1 for the Navy, by the Boston Scale and Machine Company and bolted to the outside of a Deck Gun. It held a Krag action and barrel; the trigger being 'pressed' by the plunger of an electric solenoid. This allowed cheap training.

2. Another style of device was made for the Army at Watervliet Arsenal. This also used a Krag action and barrel. A heavy collar clamped around the Krag barrel (no sights) and screwed into the interrupted breech threads of a cannon. The Krag action worked normally and allowed a .30-40 caliber round to be fired through the artillery barrel for cheap gunnery practice.