Congratulations on the promotion!!
Since you're restricted to 1 (one) new firearm, that rules out a half crate of Mosin-Nagant 91/30s and 3 crates of ammo.
I include the cost of magazines and optics (if any) in all potential gun purchases, so, going back in time (and cost)...
Midlength gas 16 or 18 inch lightweight barrel AR-15 with optics and a buncha mags.
CMP Service Grade M1 Garand with 3 cans of Greek HXP ammo and 100 enbloc clips.
Late 1880s-vintage Martini-Henry MkIV from the Nepal cache (buy it from IMA, not Atlanta Cutlery) with accessories and the stupid-expensive .577/450 ammo from Ten-X(but you reload).