I have a KKM for one of my Glock 19s--not a lot of rounds through it, but so far so good. I got it to improve ejection more than any concerns about accuracy (and ejection is stronger/more consistent now).
With an HK, I'm not sure what an aftermarket barrel really brings to the table, though--I'm guessing that you won't see a huge increase in accuracy from the factory barrel, if any.
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