Gordon Miller sells HK94's that are converted to 357 Sig he does want top dolar though.
From what I heard the 357 Sig cartridge has some feeding problems in the MP5. I have also heard some people who take a MP5 40/10 and replace the barrel with a 9mm barrel and ream out for the 357 Sig chamber.
When I had my last MP5 built I was serious considering going the 357 Sig route but decided against and went 40 S&W.
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