The current detoriation should be more harsher but depending on the weapons style.
You got a desert eagle? It should be durable. It's made of stainless steel, no need to repair the body for the next 200 years, pal.
You've got a Ak112? It's an assault rifle, atleast a russian one. It's durable as fuck. In "Real Life" you can repair that kind of rifle with the tiniest metal parts and some tiny technical knowledge. No wonder it's the choice #1 when it comes to weapons in 3rd war countries.
Anyway, if everyone's saying "Wasteland is harsh" it should be harsh. Make weapons suffer from deserts sand. Every piece of mineral from the derts sands goes into the weapon. It makes the gun unreliable within days, when it's not a traditional one. And that would also "solve" the probleme of high tier guns. The higher the tier, the faster the weapon detoriates. Look. You've got a minigun. This is a high techy mechanism. Now compare a AK-112.... You've got my point?
Weapons should detoriate A LOT more, even when lying on ground.