I got this case today in a box full of various cases:
Headstamp is “(+) FN 63”. OAL is 51mm, inside neck diameter is approx 6mm. There looks to be marks on the neck from necking down. Apart from the neck being a bit short that case is identical to a .243 Win. Was this just someone forming 7.62x51 Cases into .243 for shooting or could this be some sort of experimental round (unlikely)? This is a fired case, but is Berdan primed, so someone either got these as NPE brass or pulled the 7.62 NATO bullets from them to resize them as .243 rounds.