id of mystery ammo

[saigarocks]

i got rid of all my strange/oddball rifles and i'm left with a box of ammo i can't id length of 2 7/8 - looks like military steel - rimmed round is marked 21 and 86 - box markings - m 39 06 - 20 stuck - 74 - could be swiss or .303 - any help greatly appreciated - ron

[Rogerbutthead]

post a pic of it next to a ruler and a 7.62x54R cartridge.

[GunNut666]

According to the 7.62x54R master cartridge reference it's Polish. Pic shows copper washed steel case.