[Kos-Mos]

I would use a 500mm tightbore, add a set screw on a delrin barrel spacer and fix it at the tip of the inner.

Then make some rings from thick foam and winter boots soles (best material by far) and fit the rest of the space with them (alternating foam and soles, with random different hole diameter).

You would effectively have a very nice looking BA rifle and EXTREMELY quiet one.

Also possibly short-stroke the cylinder a bit if it is indeed designed to work with 800mm barrel (I doubt it... it uses VSR parts and a stock VSR cylinder can only go up to 550mmm past that you need a bore-up receiver and cylinder kit)