For the threaded barrel, you can either contact kwa directly for a USPC Tactical oem barrel or buy an aftermarket one like the shooters design (KWA and KSC are interchangeable). The kwa one is plastic but cheaper and you're assured it'll fit.
The threading on all HK pistols is 16mm, which means you'll need a suppressor that accomodates that or a 16mm to 14mm adaptor.
For the rail accessories, you can grab a rail adaptor for USPC, then add any standard accessories on it. Be aware that lasers are not allowed pretty much everywhere.
Yes, USPs need adaptor for everything but I love them.
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