You're kidding right? There is no way in hell HK is going to warranty a USC after it is converted to a look-a-like UMP. Nor do I suspect they would warranty any parts should they break. They are not usually just going to replace a part without wanting to see the entire firearm so they may learn what made the part break or fit the part if it needs fitting. When you undergo conversions such as this it is pretty much understood that you take on the expense of all repairs and HK, or any other company, is no longer responsible for anything that happens to the firearm.
If all the conversion was was a redo of the lower then HK "might" warranty the upper parts but I doubt it. But since most conversions also require a little work to be done to the upper too then you can bet all warranties are off.
I wouldn't really worry about it anyway. There isn't much to fail in the HK USC/UMP. Most of the parts are pretty straight forward and easy to replace if need be. Not too expensive either when you consider the cost of the gun.
Most of the parts that have me worried cost $20.00 to $50.00's in price.
Firing pin, extractor, bolt bumper etc, all inexpensive parts. Unless you shoot hot reloads and your gun explodes, I'd not worry about it too much.
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