You might want to start out with a 22lr silencer, they are cheaper and the least noisy; so if you decide that you do not like the performance, then you are out less money. Your profile says you are in the Pac NW. I hope you are not in WA, they allow ownership but use is a gross misdemeanor. ATF form 4 approval is routine and granted as long as the forms are filled out correctly, but it takes 3-5 months from what I hear now.
Any firearm shooting potentially lethal ammo can not be completely silenced by any portable silencer. You will always have some muzzle blast noise, bullet flight noise (even when subsonic) and action noise for semi-auto or machine guns. A decent silencer will lower noise by 20-30 decibels; a factor of 100 to 1000 times lower. But do not expect anything less than 115 decibels for rim fire rifles. Expect at least 125 decibels for 9mm and 130 and up for 223 Remington and higher powered rifles. As long as you are less than 140 decibels the impulse noise is hearing safe; as long as you are shooting out in the open. Using subsonic ammo in a suppressed 308 can lower noise by ten times compared to a standard supersonic load. Here is some test data; http://silencertalk.com/results.htm
While silencers that telescope over an un-ported barrel are much sturdier, the extra weight and bulk may not be worth the small gain in suppression. They also keep the barrel from cooling down as fast as an exposed one, so covering the barrel with the can might not be a good idea when shooting rapidly. Some guns with silencers that extend back are used with ported barrels. These barrels vent off some of the gases into the middle or back of the can and can lower the noise level much more. They also reduce bullet velocity, sometimes a great deal.
Adding water (oil, gel, soda) or anything else that evaporates when the hot gun powder gases flow through the silencer will reduce noise. This is caused by the liquids latent heat of vaporization absorbing heat from the gases and lowering pressure. It is a common practice when shooting pistol silencers, especially 45 acp. I have not heard of anyone doing it for high powered suppressed rifles. Using water in an aluminum can accelerates corrosion, especially if it is made form steel and aluminum.
I made silencer for my 458 socom, 300/338/510 whispers that have cover part of the un-ported barrel, while they use subsonic ammo, they do not lower the bullet speed at all. I have noticed very little in “suppressor boost” when clocking loads over my chronograph. I have noticed when I get a baffle strike though. The bullets tumble and rarely hit the target; they also leave dings in the baffles. I was lucky; I could have been picking up pieces of silencer all over the range instead. While I have not noticed any change in accuracy, the point of impact changes greatly on rifles shot past 100 yards. This is due to the increase weight on the barrel changing the way it rises on recoil and the way it whips around when fired.
If you are interested in making your own silencers, it is easy to do on a metal lathe. I learned to use a lathe by making my first silencer for a 300 whisper. I can post drawings if anyone is interested.