.177 barrels.

[Goats spudz]

Its a daisy 880 powerline multipump. The stock is missing, and it leaks slightly out of the chamber, but it still functions. Fairly loud for what I have shot. Comparable to a .22 or even a 9mm.

Wouldnt it be easier to get the barrel threaded and get a cheap crosman silencer.

[jackssmirkingrevenge]

On the bb gun I have, the barrel itself stops about an inch from the muzzle, so I would slide the silencer into that area and have an outer shell to support it on the outside of the muzzle. Kinda hard to explain...

I would chop off the front of the barrel shroud to expose the barrel and graft a suppressor to it.

[qwerty]

Maximum Muzzle Velocity with BBs: 800 feet per second
Maximum Muzzle Velocity with Pellets: 665 feet per second

So you're not shooting anywhere as fast as you think you are, that's from the PyramidAir site.
I still don't get how it can be that loud without breaking the SOS and do you really need 4.5mm tube to make a silencer? Surely you could make one out of other things?

[warhead052]

Well, I drilled out part of the muzzle (plastic part, not the barrel) and I am looking around my house for some tube that will fit, then I will drill some holes in it and put a housing on it, add some buffers and put it on.