I'm not sure if this is common knowledge and I couldn't find anything posted in the last month. It's more relevant to anyone in the early wipe stage with no access to the flea market or has low-level traders.
Once you have completed Prapor's first quest " Debut ", You can buy unlimited AKS-74U from Prapor LL1 which comes with 30 5.45 BP rounds in the mag. if you sell the gun/parts to the correct traders you spend Approximately – 14,632₽ (488₽ per round), which is currently cheaper then the BP round from the flea market at about 800₽+ per round
It's also easy to access BP rounds before you have access to the flea market/unlock BP rounds
TLDR; After completing the Quest Debut (1st Prapor quest), you can buy AKS-74U with 30 5.45 BP rounds from Prapor LL1.
See you in the Raids fellow PMCs
Source: https://www.reddit.com/r/EscapefromTarkov/comments/gwr25p/easy_545_bp_ammo/
