[Hanns]

It's stainless, don't worry about it. I've had and abused my USP since '96 and nary a hint of rust. Wipe it down and maybe a quick swipe with an oily rag if you're really concerned. Cheers,
Hanns

 

[JLStorm]

If it really worries you, you can use a Q tip and whaver cleaning solution you use on the rest of the gun.

 

[kdogg]

Toothbrush and brake parts cleaner.. They work wonders.