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I managed to get the job finished yesterday, many thanks to Duey for calling in at the right moment for the silver soldering of the bottom threaded bush to the barrel. The provisional porting is completed and the tube is packed, assembled and ready to test next week.

As well as very carefully checking for bulges and bumps as I progress a range of 2.5" factory fibre cartridges. The Subsonics can come later. I will also taking some provisional sound measurements albeit with an app on an boing. At the end of the day this is a project for myself and if it sounds quiet to my ear and others around me I have what I need.

A couple of loads:
50mm case cx50 primer14.5gn SP3 -wad as required - 9g lead shot then card and rto should give about 970fps
73mm case cx50 primer 12.0 gn SP3 gualandi micro 17 wad 21g lead shot and card rto approx 1020 fps.
Above are tested loads.
I also used these:
65mm case 615 primer 7.7gns A1 fibre wad 9 or 10 grams lead shot card and rto
65mm case 615 primer 9.3gns AO fibre wad 11 or 12 grams lead shot card and rto
Above two loads very soft shooting - dont know velocity. Came from Gualandi data.

Thank you mag41uk.

I tested the gun yesterday and with subsonic cartridges it is markedly quieter with the sound moderator than another .410 shot with the same cartridge. Disappointingly, the Eley 14 gram fibre game loads which are supersonic were bloody noisy. So two thoughts; The sound moderator works well with subs and I did manage to break clay at the end of the test session. The porting I opted for was 4 rows of 4,2mm holes over a 200,0mm/8 inch distance with several layers of aluminium mesh wrapped over the barrel to cool things down and the current design simply lacks the volume to work efficiently. All is not lost though, this gun is for Humane Despatch at very close quarters so subsonic shot and slug is probably going to work efficiently and discreetly.

Having said that, I will open up the ports very slightly and I have room to (Safely!) add more plus more mesh. I also have a stack of baffles and spacers I can add as an alternative.

Load development is complete. 2.5" case, 16.8 grams of #6 shot over some CSB5 and a tight roll crimp gave results in the 96-98dB region or not much more than a PHUT, in fact you can hear the hammer drop at some angles. It chronos at pushing 1000fps. Certainly I doubt you would know what it was unless you were quite close or in the know.

Blue case head denotes subsonic for us.

Ovenpaa wrote:Load development is complete. 2.5" case, 16.8 grams of #6 shot over some CSB5 and a tight roll crimp gave results in the 96-98dB region or not much more than a PHUT, in fact you can hear the hammer drop at some angles. It chronos at pushing 1000fps. Certainly I doubt you would know what it was unless you were quite close or in the know.

Blue case head denotes subsonic for us.

IMG_8217.jpeg

That sounds good. Would you mind showing some details of the porting and internals?

Thanks

The internals are very simple really, a threaded bush was silver soldered in place and a series of 4,2mm holes drilled at 30,0mm centres in 4 rows, offset by 15,0mm. Think of it as opposing holes every 30,0mm at Zero and 180 degrees and two more opposing rows at 90 and 270 degrees with these holes being centred between the other pair. There is a plain cap at the other end to hold things straight.

The internals are aluminium mesh from Amazon for GBP13.28 (From memory) for 3000x500mm which is enough for several goes. The outer tube is 300,0mm long and came off an Air Arms S410 carbine. Internally, the mesh is around 250,0mm long and what I did was cut it to length with scissors, then wrap it tight to the barrel until it just fitted in to the tube and cut it off. I then wrapped the mesh back around the barrel and tried to assemble everything and failed as it was unwrapping itself from the barrel. So I wound it into a tube like a rolled up newspaper, pushed into the outer tube where it opened up slightly and allowed me to assemble everything with ease as the hole in the middle of the mesh was plenty big enough to allow easy assembly.

The noise reduction with our 2.5" home built subs is staggering when compared to factory subs, possibly 2/3rds as loud although I need to put it back across the meter to get some numbers. I can definitely hear the hammer drop at 90 degrees to the gun at 2m with our loads and I doubt you would know what was going on at the other end of a tennis court.

So simple and yet so efficient, although I do believe this is down to the cartridges as much as the silencer and oddly, the tension on the crimp is as important as anything. We did get some loads down to 700fps with no signs of over pressure so dependant on temperature I can always slow things down a bit more. Regardless, 16,8 grams of #6 shot at 1000fps ish will do a lot of damage at 25 metres.

Final pictures:

Ovenpaa wrote:... and I doubt you would know what was going on at the other end of a tennis court.

"Hello ? Ladbrookes? It is ? Good. Badger here. We'd like to place a wager on the outcome of next year's Wimbledon Championship please..."

Ovenpaa wrote:The internals are very simple really, a threaded bush was silver soldered in place and a series of 4,2mm holes drilled at 30,0mm centres in 4 rows, offset by 15,0mm. Think of it as opposing holes every 30,0mm at Zero and 180 degrees and two more opposing rows at 90 and 270 degrees with these holes being centred between the other pair. There is a plain cap at the other end to hold things straight.

Thanks for sharing. That is twice as much internal porting than the .410 Mossberg. But that does have the moderator tube overhang the muzzle by 20cm or so.

Think I will look at putting a few more holes in the barrel at 90 degrees to the originals as you have.

Might need to look for a .410 loader too as I have only so far tried factory subsonic loads...