12.7mm rounds are ammunition found in Return to Castle Wolfenstein that is used by the Venom gun only. They are always found in small cases that give 50 rounds. Their first appearance is in fourth part of Mission 4: Secret Weapons Facility as the player will find their first Venom Gun. The 12.7mm rounds are quite uncommon until Mission 5, therefore it's almost impossible to reach carrying limit of 1500 rounds. Unless, you did not used it any of enemies except only for the Boss itself, or at the most of the time: overkill. The 12.7mm rounds in real life is much bigger than the in-game counterpart. Cailbers like 12.7mm are mostly used as anti-material role rather than use against personnel. It is possible these are 7.62mm mislabeled as 12.7mm rounds.