Suppressors are a great choice for home defense, IMHO. If you do a search on youtube, you'll find an interesting and informative video posted by a gentleman who demonstrates the effects of firing a couple of different suppressed handguns in his home (he shoots down his hallway out an open window into the woods behind his house after verifying it is safe to do so)
Yes, a single suppressor may be used on multiple firearms of different calibers: provided, of course, that those firearms are of the same or a smaller caliber as the one the can was designed for. For example, you could shoot a 7.62mm can on a 5.56mm or 6.8mm firearm provided that the suppressor interface (e.g. threaded muzzle, proprietary mount, etc...) is the same. Just don't expect a larger-caliber silencer to be as quiet on a smaller-caliber weapon as a silencer made specifically for that smaller caliber. Interestingly, Silencerco is coming out with a rifle suppressor called the Saker that will have the ability to accept adapters to allow for several attachment options with one can.
The same applies to handguns, with one small difference. You could shoot a .45 can on a smaller-caliber handgun like a .40 or a 9mm, but you're going to have to swap the silencer's piston with one meant for the smaller caliber in order to get the gun to cycle. Again, don't expect a .45 can to be as quiet on a 9mm host as an actual 9mm can. This is due to the fact that the exit hole on the can is much larger than the projectile, and thus allows for more gas to escape.
Most .22LR cans are rated for use on .17HMR and .22 magnum hosts, and some are even rated for 5.7mm.
As far as mounting a suppressor on a handgun, there are lots of after-market threaded barrels available. Some may be "drop-in", but I believe that having to do a little fitting is the norm. There are also companies that can extend your factory barrel and thread it for a suppressor.
For rifles, depending on the weapon and the suppressor you have a few options. A gunsmith may be able to thread the barrel, or you may be able to fit your gun with a quick-detach muzzle device if the can is designed to mount on such a device.
You are absolutely not overestimating the potential for hearing damage. Gunshots are LOUD, and firing any weapon in an enclosed environment such as your home is extremely dangerous and likely to result in permanent hearing loss. Eardrums can be ruptured, and the effects of the concussive blast will be disorienting to you and anyone else in the vicinity be they friend or foe. The flash can also be blinding to your dark-adjusted vision. Suppressors are extremely effective in reducing the noise, concussive blast, and flash of gunshots, making them ideal for use in home defense. It's still going to be fairly loud, and may ring your ears, but it'll be far less likely to permanently damage your hearing or temporarily blind you.
Hope this helps!
Last edited by TennTexan; April 30, 2012 at 02:27 AM.