i had a gun power stealth for which i bought a massive silencer, looked fantastic, i thought that with the size of the silencer it would be more than capable of trapping the blast of air and quietening the stealth, it did not work! the internal volume of the (so called) silencer was that big it caused an echo, increasing the blast sound rather than muffling it!!!
on the other hand a former work mate made a silencer for his air gun, at first i didn't notice it on the gun, on a closer look it looked like a barrel weight, until he pulled the trigger, all you could hear was the faint click of the action, he said that the hole in the baffles was just (and he did mean just) bigger than the pellet.
in ww2 the welrod pistol had rubber washers with the central hole smaller than the bullet
http://www.timelapse.dk/thesilencer.php to keep as much gas in the silencer for as long as possible
things to think about when building a silencer
have a stiff rod the internal size of the barrel (.177 or .22) to aid the lining up of the baffles
build the silencer on the gun, this will insure the baffles are aligned correctly when using the rod
work out how much air the gun releases in to the barrel, then you can use this to work out the volume the silencer has to be (too small and the silencer will be overwhelmed and could be dangerous, too large and it will echo) it also means that the silencer is as small as possible
make the holes in the baffles as small as possible
you will have to experiment with how many baffles you need for your gun