- The FN 509® Is Built On The Proven Architecture Of The FN™ Compact, FN Made Changes Internally And Externally To Meet The Rigorous Performance Standards Of The MHS Requirements And Further Developed The Platform Into The FN 509® With The Help From Industry Experts. Over The Course Of Development, The Platform Has Been Tested Extensively For Reliability, Ammunition Compatibility And Durability – Totaling More Than One Million Rounds.