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From yahoo answers, the guy seems to know what he's talking about.
I think the Yahoo Answers quote is from one of my answers on that website. While I try to be as accurate and informative as I can, I think using the Yahoo Answers website for info on firearms is not a good idea.
The best forums for questions on title 2 weapons are http://www.silencertalk.com
. The subguns website has an excellent FAQ here; http://www.subguns.com/laws/laws.htm
The ATF website is the best resource, but is not all that user friendly.
In my opinion the info offered so far is good but I have a few comments. Silencers work very well on semi-auto pistols as long as they are recoil operated. While significant noise still comes from the action, a good silencer will make it safe to shoot without ear plugs. I have not shot my Makarov with my 9mm can yet, but I suspect the blow back action will allow too much noise to shoot suppressed without hearing protection. Blow back operated 22lr pistols are suppressed well with small silencers.
Silencers also work very well on gas operated rifles like the AR-15. While they are louder than any bolt gun of equal ballistics and barrel length, the change in noise level is very dramatic. You have to be present while shooting to compare suppressed and unsuppressed fire. Camcorders and TV/PC speakers do a very poor job of recording and playing back the difference in noise levels. I have been told that the M-1a and AK's which vent gases from the action differently than the direct impingement AR's are louder. I will find out after the ATF approves the form 1 for my AK can.
The 1895 Nagant revolver seals the gap by moving the cylinder forward when cocking allowing the brass (longer than the cylinder) to enter the back of the barrel. Single shot firearms are also very well suited to suppressor use as not only do they have no action noise, they are several inches shorter than most center fire bolt action rifles and are less cumbersome when fitted with a silencer that extends OAL by 6-8 inches.
In my opinion silencers are very well worth the hassle and cost. I make my own on a metal lathe for $20-$50 in material plus the $200 tax. Even though I can not use them in WA, I take some with whenever I travel to a gun friendly state and enjoy them very much.