Mostly throwing down the "fightin' words," really... I like my MAS-49, but Marianne also "poops where she eats" as a direct impingement system (the French were experimenting with direct impingement before Eugene Stoner was a gleam in his daddy's eye). I could probably subject her to a "gasoline and grease" diet and she'd function fine... but I care a bit too much about her as "living history" for that.
But getting back on topic... My vote for dirtiest ammo goes to Barnaul, specifically in 9x18mm Makarov. Not only with leaving the firearm dirty as all heck, but with raising an unholy stink and making my eyes water even in decent ventilation. Hardest box of 50 rounds I've ever been through (I was stubborn to get my money's worth out of 'em), and when I got back to the house, my roommates were "Man, what's that stench?"