Not convinced it's critical or needs to be tight - all that a silencer does is slow down the exit of high pressure air, I don't see the relationship between pellet size and exit hole size, it would appear that the muzzle blast has the potential to overtake the pellet anyway. The airflow is almost certainly self disruptive within the chambers, particularly the first one, i.e. it prevents (delays) the exit of high pressure air even with simple disc baffles (and very much so with K baffles). What the first chamber won't do, the second certainly will. I remember having no patience to test a new mod I made for an S10 (Fac, so plenty of air!) so fitted it without any internals at all (but with end cap). It worked amazingly well. Volume i.e. size is the main factor, if a moderator is small then air will still exit it at some pressure, and this makes the mod inefficient. One of mine for .22 was made using washers for 8mm bolts, so probably around 8.5mm. Just to add that the service parameters were well beyond FAC... and the result was silence that the impact at target was louder than the sound at the muzzle.