If you want an advantage or new experience:

(for the M-60) were popularized shortly after the game's 2010 release, modern players almost exclusively use downloadable tools for single-player enhancement. Review of PC Cheat Options

Since you are likely playing the campaign offline, using "trainers" or external software is the standard method for PC.

In the older retail versions, entering codes in the "Unlockables" menu granted early access to specific gear: try4ndrunf0rcov3r : M60 Machine Gun your3mynextt4rget : QBU-88 Sniper Rifle cov3r1ngthecorn3r : Uzi aw3somef1rep0wer : USAS-12 Shotgun 2. Multiplayer: The "Project Rome" Era