Could also be a "sub-caliber device" wherein a rifle, using blanks, was fitted into the breech of a cannon, to simulate firing the piece during drill.

You may have the remnant of a sub-caliber device. 

The cartridges used were not necessarily blanks.  Some were loaded,first with 220 grain bullets and after about 1925, with 172-grain M-1 bullets.  These bullets were loaded into regular Krag cases, but with odd primer pockets.  I do not know the load or velocity of the loads using 220 grain bullets.  Maybe they were just standard loads.  The loads with the pointed 172-grain bullets were supposed to give an instrumental velocity at 53 feet of 1,960 fps.

The primers were often large, odd sized protected primers.  Some sub caliber devices used a regular Krag action, but other types used the big gun's firing pin and that really smacked the primer. 

The sub-caliber devices were inserted into regular artillery pieces or naval guns and were fired on some military rifle ranges, or maybe out at sea.  The idea was to give the gun crews experience with the gun's sights without using the big shells.

In 1962, I was part of a group of ordnance-rated sailors taking inventory at a US Naval Ammunition Depot.  I inventoried a few cases of "cartridges, sub-caliber, cal. .30 M-1925" with dates from the 1930s.  I asked one of the old-time Chiefs about them.  He explained what they were and how they had been used. 

We serviced older naval craft, including submarines, belonging to Chile, Peru & Ecuador.  They had 3" deck guns.  Maybe the old sub-caliber cartridges were for them.