50 Cal. Legth 20.30mm/1.97 In. Thanks Keith
It looks very similar to a U.S. Navy Mark 15 combination primer, but with the base of the case where a primer would be seated (in a regular cartridge) bored out.
Check here for a drawing of one of those. Many other primers are listed in the menu to the left.
Well done to Bulletpicker for posting his site!
It appears to be a different round. The inside lay out looks differet. NO headstamp.