Tough situation living in the city with limited space. I feel for you.Using different length spacers and washers in the empty space in the shroud should quiet ig down a little more. Just be careful to make sure everything is centered to the bore. Otherwise there will be clipping & accuracy issues.Another way to go would be to turn the power down if you can since you already have limited space you dont need high velocities. This would increase the shot count & lower the noise.
I wouldn't turn down the power any since its already a low power 10m target rifle and adjusted from factory as such.First try just a cap over the end of the shroud with a hole in it in the correct location. 8.5" is alot of space for a low power .177. Do you know if the barrel is vented to the shroud behind the muzzle?Then maybe try additional washers and spacers.Some guns w/ built in moderators, such as FX 400 have no baffles just a hollow air space to absorb the blast.I checked out pics of your gun on Pyramyd... beautiful!
"IF" barrel end LDC comes off and end cap can be removed ... YES it can be made much more effective.Touchy subject and we can't speak openly in the forum about such things, So ... If a mechanical type the info on what to do is out there, if not PM me and can advise in private.