Re: HK P7 K3
There is a thread on silencertalk.
AAC had some 9mm barrels with unusual threading but the big challenge was to find someone to pull the old barrel and replace the new one by press fitting it without cracking the frame.
However, your K3 has a barrel that is easy to remove, bu loosening the barrel nut, so my suggestion is to contact tornado technologies and see if they will thread a barrel extension for you.
I had a p7M13 and would love a P7K3 in .22 - wherever did you find one?