The HK USP uses polygonal rifled barrels which could prove to be problematic with soft lead bullets if not cleaned properly. Enough lead build up in the chamber could cause an out of battery Kaboom. This has happened in various pistols other than and including HKs. Other shooters claim lead bullets are not a problem in a HK or Glock polygonal rifling as long as proper clean-up and maintenance is completed. I tend to agree with the latter camp. However lead bullets are not created equal. Those lead wire extruded soft lead bullets (Hornady) are made for target velocities and were made to be produced cheaply and quickly. I think Dan Aykroid from “Saturday Night Live” used the term “mass quantities” but referencing a different context. These die extruded lead bullets have a tendency to lead barrels because of their hardness (or lack of hardness). They are super soft, but super cheap (like some shooters on this forum).
Hardcast lead bullets are a different story and can be produced to withstand magnum velocities and powders without leading a barrel. Another technical component with leading has to do with the true diameter of your bore to bullet diameter. You need a good seal (tight fit) when shooting lead bullets in any handgun or rifle. It’s not good to get those hot expanding gasses around your bullet when trying to drive it down your barrel.
Why not just buy coated lead bullets? That is why they were invented. To shoot low cost lead bullets through polygonal barrels (Glocks) (HKs) and (Walthers). Competition shooters use them in the action pistol sports. Many bullets casters make a variant of coated bullets. You can even get them in pretty colors like green, blue, black and even red. The red colored ones have been dubbed “lipstick loads.” Federal has the Syn-tec line of factory ammo with a synthetic coated bullet for non-reloaders. Reminds me of the old Federal Nyclad of years gone by. Supposedly, the NyClad was developed to keep the lead dust down for LE indoor qualifications. Less lead, what could be bad? They were also an inexpensive duty load which was simply a soft lead hollow point covered by a nylon coating. I believe they loaded them in 38 Special, 357 mag, and 9mm. Low tech, but it worked. If you wait long enough in the firearms industry, what goes around, comes around.