Calibre means dia. of bullet, not cartridge ? (Please forgive the beginner here). The Minie ball is reported as .771 calibre - I have a round musket ball that matches that measurement - these two finds are .600, or 15mm +- due to distortions. I'll agree, they look like Minie's, but seem a bit...
This should be an easy one for the experts amongst you - I have been given a couple of 'bullets' to identify, they are 15mm dia, 25.5mm long with a 9mm hollow at the cartridge end, weigh 31g and are lead. They were found deep in the ground, close together. It's a long shot - literally - but...