How much louder is a one or two caliber up muffler, compared to the purpose built one? When does it matter; and why? Rather than quoting some perceived authority or being adamant, I thought some pictures might help.
The attached images show three styles of muffler. One has flat baffles like many DonnyFL models. Another has cones - also very common. The third has a narrow perforated tube, feeding one or more partitioned spaces - in this case two. These latter tubular types are not very common for PCPs, except for "hair curler" mufflers, with much larger bores.
The smaller the bore size of the perforated tube type, the less air or gas escapes past the projectile. This is because that small ID exists all the way down the muffler bore axis. So, smaller is better
with this configuration, up to the point where misalignment with the barrel axis can cause the projectile to contact the muffler bore. This long tube type can also aerodynamically steer the projectile, if it is "too tight". That can move the point of impact on target in a less than useful manner, but that is not the main subject of this discussion.
Mufflers using flat or conical baffles have a very short sections of "tube", with long stretches of open space between them. No matter if that space is cylindrical or conical, the air or gas not only can enter that open space radially, we want it to move away from the central bore, so less air or gas is lined up with the bore axis, to exit down the main bore of the muffler.
If you look at the images, it is only when the projectile is travelling through the narrow section of bore of the baffle, that the bore to projectile clearance makes any significant difference with regard to how much air or gas can leak past the projectile, to take a short cut down the bore.
For most of the projectile travel with flat or conical baffles, there is no "bore diameter" near it. I suggest that for DonnyFL style flat baffles, or Marauder shroud conical baffles, bore size matter much less than the blanket statement we often hear or repeat, would suggest:
"Tighter is better"
"No more than 0.02" radial clearance"
So, the answer to what happens when you shoot your .177 through a .25 muffler is; it depends on the architecture, but generally, very little is lost. Just because your meter shows a 2 dB difference is no reason to lament. Most humans can't detect sound level differences of less than 3 dB, and if you think you do, it is probably the tone that is different. I actually think that how pleasant a muffler sounds is more important than the exact dB it measures. The less is sounds like a gun-shot, the better.
I design mufflers with a 1 mm per side clearance on the projectile diameter; then increase the bore clearance from the back towards the front by 1/4 degree per side. So, comparatively "oversize" clearances on small calibers. This very generous clearance is enough to make some people weep. Others like the way my muffler sound and shoot. Many of them measure greater dB reductions than expected with such loose bores. Go figure.
instead of a fight of all of us fighting united against shot noise. 
