Current Lapua rimfire ammo lot numbers are composed of eleven (11) digits, five (5) digits followed by a forward slash, followed by another six (6) digits.

The five digits before before the forward slash:

  • First two digits (digits 1 and 2): Prepend a ‘3’ in front of the digits to get muzzle Velocity in meters per second. Example: If first two digits are 27, then the muzzle velocity is 327 meters per second.
  • The last two digits (digits 4 and 5): Year of production code. 54 = 2019, 55 = 2020, etc.

The six digits after the forward slash is the actual lot number (assumed to increment sequentially).

Reference: The Shooter’s Mindset podcast episode 299.