The 16J / 12fpe converted Walther LG400 is not as loud as a Steyr LG110 (these are really noisy rifles) but still making much more noise than an Airarms EV2 or S400. And at my training location FT Schalkhaar the noise of the LG400 makes the farmers dog (which is about 50 meters away in its kennel) bark once at every shot.
The long 15mm inner diameter carbon barrel shroud on the LG400 has space enough to accommodate a sound moderator. I took some 20mm diameter POM / Delrin rod of 150mm length . Reduced the diameter to 14.90mm. Radially milled 8 holes of 12mm. At the lathe I made two 1.70 mm deep and 2.5 mm wide grooves at both ends for 10mm ID x 2mm CS o-rings. The o-rings center the moderator in the barrel shroud. The inside of the carbon barrel shroud is quite rough and the o-rings have enough friction to keep the silencer from being blown out by the air pressure.
The bore of this sound moderator is 7mm. The front end of the bore is threaded with M11x1 to be able to pull out the silencer/moderator with an bolt that can be screwed in. (I just had this M11 bolt and tap lying around. M8 or M10 would work as well). The rear end (barrel side) is opened up to 12mm for the first chamber and 10mm for the second chamber. After that the bore is 7mm. This is the why the o-ring is not at the end. The groove for the o-ring would have cut the moderator in two pieces.
The noise reduction with this moderator is really good. The dog stopped barking 🙂
I tried to measure the sound level with and without moderator using a microphone on the computer and Audacity software but I did not get good results. Looks like the microphone is much too sensitive and gives to high a signal even with the moderator in. I tried this indoors so I guess there is too much echo or so. But the barking has stopped so I have reached the sound reduction that I wanted.
Accuracy? Yes the LG400 with moderator is still accurate. Accurate enough to shoot 43 points out of 50 in a windy FT Match where the winner shot 48 (match results here: http://www.ftschalkhaar.nl/?p=2672 ). I tried to test for accuracy with the moderator but it was quite windy during the last few training sessions. But I easily managed to stay within the 40mm hitzone at 50 meters when my wind correction was okay. And the vertical spread was minimal, about 20 to 25mm @ 50m. This gave me enough confidence to use the moderator during the match.
Update March 2016: I have now shortened the moderator. The 150mm length shown here is not necessary for a good reduction in noise. I’m now at 50mm length with 3 holes and this reduces the sound just as good as the long one. Maybe even better. This is probably caused by the larger volume of air inside the shroud that can absorb more pressure.