.303 blank or grenade cartridge

I have a cartridge in my collection that could be a blank cartridge or a grenade cartridge. The headstamp is F N 57. I personally think it’s a blank round because when you shake it, you can hear the case is only filled with very little propellant. I think to little for a grenade cartridge…
who knows the answer?

I agree it seems to be a normal blank. I have the same cartridge and it seems to be an FN .303 version of the M1909 .30-06 blank.

FN grenade blanks that I have all have a rosette crimp.