The .380 is similar enough to 9mm that you can easily and safely use any 9mm silencer for a .380. I don't know if a specific silencer exists for the .380, but if it does, don't buy it; just buy a 9mm.
As was mentioned earlier, the Walther PPK is blowback, and not tilt-lock. Because of this, if you put a silencer on it, you will get a lot of back-pressure coming out of the ejection port. You can get around this by stuffing in a stronger spring, but this will take a minor bit of trial-and-error. Also as stated earlier, you will not need a 9mm with a booster because the gun doesn't have a tilting barrel design (its also why a .380 tends to be more accurate at shorter distances - well - that and recoil). Anyway, until you get that spring issue fixed, you should fully expect that the .380 blowbacks will be louder than a Sig 9mm, or a GLOCK 9mm, or a Beretta or whatever.
That said - the Walther is the
quintessential gun to suppress, but I personally would seek out a FireStorm/Bersa .380 just for price difference. I'd also look at Tornado Tech for the threadjob - which will be a 'sleeve', as opposed to a simple 'thread'.http://www.gunblast.com/Bersa_Thunder380.htm
I believe Lipsey's also sells the gun with unusual gold highlights.http://www.gunblast.com/Bersa_Thunder380-2.htm
Bear in mind that my preference for the Bersa has absolutely nothing to do with disatisfaction with the Walther, its just that this type of gun is very simple, and very, very similar models can be had for much lower prices. I also like the Bersa because it can be had in the exact same style but in .22. However, my complaint on the platform is that in addition to being slightly harder to suppress, the ammunition is shot-per-shot more expensive than 9mm. So more expensive, louder, less powerful.... does it at least have greater capacity?
Doesn't change the fact that I want one, and have had a helluva time talking myself out of it for the last three years.
Not if, but when
you get your system up and optimized, please come back to this thread and post pics, a timeline, what challenges you had, and what answers you developed. Like you, I also think that this combination (especially with a Thompson Poesoiden) is THE best rig - not the most practical by a long shot - but a sweet little rig none-the-less.