The official read from the BATF is that a silencer is a silencer and it regulated by virtue of it's inherent ability to reduce the report of a firearm--NOT by reason of what it is currently attached to.
SO, the only way to be truly safe is to be 100% sure that the silencer can't be removed from the airgun and adapted to a firearm. Some airgun manufacturers have designed silencers that are completely integral to the airgun in such a way that it would be impossible to remove them without destroying them. That's the safest way to go.
Pure pneumatic airguns, that is, guns that work on gas pressure alone (Pump, precharged, CO2, etc.) are going to be louder FOR A GIVEN POWER LEVEL than a decent quality spring piston or gas piston gun.
I own a .20 caliber R1 and a .20 caliber Sheridan. Although the R1 is about 200fps faster with a given pellet, the Sheridan is louder.
The reason is that the virtually instantaneous compression of the air inside a spring-piston gun drives the temperature up. That, in turn increases the pressure, and you get this nice dual-effect physical phenomenon that puts pressures through the roof. The pressures and temperatures get so high that you can't use petroleum based lubricants in the compression chamber or they will explode (diesel) during the firing cycle.
Anyway, as soon as the pellet starts to move, the pressure starts to fall. That brings the temperature back down, and the dual-effect phenomenon works the opposite way as the pellet goes down the barrel. The result is that the pellet gets a tremendous initial "boot in the butt" but by the time it exits the barrel, there really isn't a lot of pressure behind it. Low pressure at pellet exit means low muzzle blast.
Of course, there is a lot of mechanical noise. Even a nicely tuned springer makes a good bit of noise during the firing cycle. Tuned or not, there's a lot of metal moving around when you pull the trigger. I've heard experienced airgunsmiths estimate that only about a third of the total noise of firing a springer is due to muzzle blast. In a lower powered gun such as the R7, it's probably lower than that.
Airguns are penetrating weapons. They don't have the energy to shock anything but the tiniest targets, they merely poke holes. But, they are good at poking holes. AND, it turns out that if you are a good shot, and you can poke holes exactly where you need to, you can kill surprisingly large animals.
This is part of why I get such a big kick about the caliber arguments. The fact is that it really IS all about shot placement. Even an airgun will do the trick if you make the shot. There's a guy on another forum who will tell you that the only reason he's alive is that he was carrying a 10mm pistol (instead of a 9mm, 40S&W or .45ACP) when he was attacked by a pack of dogs. Too bad he's not going to read this post--I'd like to know what he thinks of an airgun for use against large dogs...
For the record, even a high-end magnum gas-spring gun like the Crow Magnum (Theoben Eliminator) is a total wimp compared to even a .22LR.
