There are many things I know little about, rimfires being one of them ;-) so any help ID’ing this will be appreciated.
Lead bullet, head and lower part of the case is brass and the top half is copper. The headstamp is GG.
OAL is 13.4mm/.526"
Case Length is 7.45mm/.293"
Bullet dia is 5.58/.219"
Rim dia is 7.14mm/.281"