I was impressed with the simple design of the internals on the USP and I was also I pressed with how quickly I did it *pats self on back*
I've always found HKs relatively straightforward to work on. They're nowhere near as simple as a Glock, but after one or two times it's pretty easy.
Know the status of your weapon
Keep your muzzle oriented so that no one will be hurt if the firearm discharges
Keep your finger off the trigger until you have an adequate sight picture
Maintain situational awareness