I appreciate all the advice, but I'd still like the original questions answered.

There is no "inner sheath" on an airsoft silencer. It's basically a hollow tube with foam inside. Nothing more. Use whatever you want for the outer tube as long as it's solid.

I recommend that you take a look at music (instrument) store for drum felts (the kinds used to cushion cymbals). They're cheap can be used for inner padding for the silencer tube. They already have a hole in them. Just stack a bunch of them. As for the hole, generally it's made to be slightly bigger than the size of the caliber of weapon you're replicating, and not based on the actual BB size. For example, I have a supressor for my MP5 which is normally 9mm. The hole on the supressor is 14 mm.

And remember, the silencer has to be PERFECTLY parallel to the bore of the gun, or BBs will bounce off the inside of the silencer.

Crunch you've been spending too much time in sound studios :)

Indeed. My home studio also doubles as my basement firing range. Got the idea of the cymbal felt when I saw one on the floor one day, looked at my suppressor, and realized it would fit. I opened up my suppressor, and lo and behold, it was the same size and the felt was of very similar composition to what was in my supressor.

An airsoft "suppressor" is not much more than a decoration. If it actually reduces sound level it can create a lot of unintended problems... at least down here in the States it can get you seriously arrested unless there is no imaginable way to make it functional with RS...

The easiest way to make sure it can't function with RS is to make it part of the inner barrel. If you would have to cut it up and re-manufacture it to make it work with RS then it will probably pass... and then again if you ask two different people in LE about this, you will likely get three answers. The subject has lots of politics and interpretation to it. Besides, it can give you the opportunity to use a longer inner barrel while making it look like a can.

Myself... I'd get a longer inner barrel and build it around the area that is different in length, or get a shorter outer barrel... or just build it into the outer barrel in some cases.
For the sound level of an airsoft gun, a bit of room for diffused expansion is more important than foam as a sound reduction... Keep in mind that the noise level of an airsoft gun is about what a suppressed RS arm is, more or less. Even if you get the gas nicely quiet, the blow back mechanism is still going to make noise. For the distances in airsoft, that is still going to be audible at most game ranges. I will just be less easy to tell exactly where the noise came from.

I would make it just thick enough to be durable of whichever you can manage most easily. Metal cans will possibly dent, and plastic won't but metal will also take more punishment without failing. Plastic is lower strength but in a lot of cases more forgiving... until it gets whacked hard enough to crack it. I'd personally go with whatever the outer barrel is made of. If you have a full-metal gun, for heaven's sake put a metal can on it to keep it looking totally tits. Making the can out of metal when your outer barrel is plastic really doesn't provide any advantage.

The thickness depends on what material. For a DOM steel you might get away with 0.025" (0.64mm). For aluminum or brass about 1.5 times that thick. For something like glass filled ABS something like that a minimum of about 0.065" (1.7mm) and for lesser plastics like unfilled PVC I think something like 0.080" (2mm) would be more appropriate. BTW... filled and unfilled refer to the presence of fibers in the plastic resin not the foam.

Unless you have a machine shop, you would probably be wise to settle for plastic... you can manage a cleaner job with less fuss with plastic and it's much less expensive than DOM steel.

If you're going to use foam, and you don't have to... and in some places like here in the States that would get you a serious hard look... Then use a pliable foam not one that sets up hard and brittle. A foam with some give to it like a latex household sealing/caulking foam will absorb sound better than a foam that gets completely hard like a cyano-urea type insulation foam. follow the can directions with the expectation of slightly overfilling and trimming rather than guessing and falling short.

BTW... using a soft foam inside the receiver is also a great way to quiet an AEG, where the piston makes most of the gun's noise.

The foam used for theatrical foam appliances (noses, cheeks, Klingon foreheads... etc) is superior to most stuff you can get at the hardware store.
It's soft and pliant yet easy to cut with a sharp knife after it sets.

I believe his barrel is plastic ,from what he states in his review ! I don,t think your going to have much luck mounting a suppresser on that thing !

Not really a huge issue up here. We can't legally manufacture them but I haven't heard of anyone getting in shit for having one on their airsoft gun.