My mate who is on holiday in Hungary asked me what this is ? No, he didn't take a photo of the base. I suggested 20mm .
knockout in round 5 if he is a giant it could be an 8 inch shell if he is of a smaller stature it could be .22
I'll take a WAG and say 20 x 110RB based on shallow shoulder angle and crimp ring just below the projectile.
2cm for MG FF probably any beginning on page 95 here: https://www.lexpev.nl/downloads/handbuchderflugzeugbordwaffenmunition19361945.pdf