I actually have a Gamo .22 cal break barrel right now for the purposes of varmit control in the back yard. It’s a smooth bore that fires both pellets and shot shells.
It does not have a muzzle break, but I have noticed that if you fire it without a pellet, it is louder than a .22 rifle. However when fired with a pellet or a shot shell, all you can really hear is the the sound of the spring decompressing. I have fired it from within the house into the bird feeder (at pesky squirrels who refuse to eat the seeds on the ground that I put out for them), so it seems impossible that my neighbors will hear it.
The only thing that I do not like about this Gamo break barrel is that it only has a shot gun bead sight. It is very hard to be accurate with a pellet with that sight and the smooth bore barrel. I bought a 25 $ cheapy laser sight for it, but the spring decompression actually produces a kick that is strong enough to un-sight the laser every time it is fired.
Oh and the shotgun shells are deadly for Mr. Squirrel, but they are quite expensive compared to pellets. I do love my Gamo, but I’ll never forget that Benjamin Sheridan from when I was a kid.
--if you can hit a grasshopper offhand at twenty-five feet you are deadly on anything "real"--