Guyra108,293 Locked 07 Oct 2021 07 Oct 2021
The first round of a classic CS match is called the "Pistol Round" because you can't buy weapons aside from pistols. Just win 250 of these in either Competitive or Wingman mode, and the achievement will unlock.
NOTE: For some reason, the first round in Flying Scoutsman mode also seems to work for this!