I'm not sure that a UMP barrel IS chrome lined. It's not mentioned on the HK website, nor in any of the manuals. The twist rate and direction, and number of grooves is specified, but nowhere does it say chrome lined that I can find.
If you have an actual UMP barrel, I'd use that. (And I do as well.)
And neither of them are stainless steel, they're parkerized which doesn't work on stainless. The factory barrels are all alloy steel, possibly 4130 or 4140, or some other high-carbon steel. There are a lot to choose from.