Prapor — unlock Reserve (5 dogtags + Shoreline extract)

Two parts: collect 5 PMC Dogtags and hand them to Prapor, plus survive/run-through Shoreline. Dogtags drop on every PMC corpse — both BEAR and USEC count, both factions count regardless of yours. You don't need the kill credit; loot tags off bodies you find that someone else killed. FIR is NOT required, so you can buy tags off the flea market if you're impatient (cheap). The Shoreline survive can be done in any single raid; combine it with Witness's Shoreline tape (Kozlov's hideout) or Falling Skies' Shoreline drops if you have those open.

Unlocks Reserve, the high-tier military map full of Scav Raiders and great loot. Save Reserve for after you have decent gear — Raiders are aggressive and scary for newcomers.