LoL, no wonder it is louder than you expected a silencer to be!! I have never seen an HW100 that did not have a threaded barrel, maybe some markets don't allow this so versions like yours are produced for that market?
Anyway, yes buy an adaptor to mount a silencer, a 16mm one should be ok, it wont wobble once tightened, but it will be a smidge off centre, this wont matter as the silencers have enough clearance to be used on larger calibres and I have seen barrels with the bore offset by more than that dia difference.
The Theoben Rapid has a 15.8mm dia barrel, I wonder if a slip on adaptor from one of those would fit?
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.