There is no "rust" mechanic, mode, or glitch in Call of Duty: Modern Warfare 2 (2009) on Xbox 360. You're likely describing one of these:
Cause: Skill decay from not playing.
Fix: Warm up in Private Match / Free-for-All 10–15 min. Focus on centering, pre-aiming common angles, snap drills.
Rust is a famous small map in MW2 (scrapyard, two towers). If you mean "how to get out of the map Rust" → you don't; it's a standard MP map. If you're stuck in geometry, it's a glitch—kill yourself (grenade) or wait for round end.
"Rust" isn't a thing you "get out of." Define the actual symptom for a real fix.