Martin-Baker ejection seat case. 2 1/2" long x 1 7/8 across base. Berdan primed. Base of case marked; SEAT EJECTION PRIMARY / No3 MK 3 / MBEJ-646-AVR / MARTIN-BAKER AIRCRAFT CO LTD / MBAC 20 (in circle). From some brief web perusal; this is a blank round used to eject the aircraft seat from modern(post WW2) aircraft. The MK 3 was used in such aircraft as the Javelin. There were 2 primary cartridges and 4 secondary. When fired they caused metal posts to telescope out and project the seat and it's contents up and away from the aircraft and its appendages(or empennages).