One advantage to a permenatly attached suppressor is, it can be only "one" tax stamp on a SBR. You still have to pay the tax, but you dont have two pay one for each, as long as the can makes the "barrel" longer than 16", and is permanently attached.
Which isn't considered a silencer because it's on a muzzleloader (which isn't considered a true firearm to begin with under the law).
SilencerCo also makes the Maxim9 pistol which has an integral silencer. You still have to pay the money and wait just like any other can.
You really need to watch things when it comes to stuff like this. Back in the early 2000's, my son bought a paintball gun off a buddy, and it came with a homemade, removeable "suppressor" on it. A couple of pieces of PVC pipe stuffed with Chore Boy scrubbies. I was telling my buddy, who was an FFL, and he tells me its still considered a suppressor by the ATF, and showed me the cite in the rule book. Kind of silly, but there it was.
My BIL buys a Gamo air rifle, with a permanently attached suppressor on the end of the barrel, and it is legal, as long as you have to destroy the barrel to get it off.
Both are nonguns, but one is legal, and the other not. Just more typical nonsensical rules from the ATF. And rules that can/will get your ass in a wringer, if youre not careful.
I know very little about them but we had a guy at the club that was shooting a .223, single shot pistol of some sort at 100 yds. with his suppressor. He was shooting around 1/2 inch groups and the sound was like a .22 rifle, from my memory. I purposely removed my muffs to see.
I have an AAC M4-2000 suppressor from my 5.56 AR's. According to AAC, it eliminates something like 95-97% of the muzzle blast, and I pretty much believe them. I can shoot it from my carport, without ear plugs, and all you hear is the crack of the round going down range, which can vary with distance.
I cant shoot a .22 rifle from the same spot, without losing most of my hearing for a couple of days. Thats how bad my ears are.
One other noticable thing with the suppressor on is, you can actually hear little critters "pop" when you hit them. The first time I shot a bunny in the garden, there was a strange pop, like a bassy balloon popping. I thought at first the round went through and hit something behind it. When in fact, it was the bunny exploding from a Nosler BT running around 3000fps.
Ive also shot my Glock 17 in the same spot dispatching a critter, and my son, whos room is right off the carport, and has a window in it, thought Id "dropped" something. He didnt think it was a shot, when in fact, it was two quick shots. And he was right, I did "drop" something.
The way I equate the sound of subsonic 9mm coming out of my AAC Eco-9 that has ablative added, is more like a heavy car door closing, or maybe a air nailer. A lot depends on conditions and its not always the same. It usually doesnt sound like a "gun shot" tough, which is really what the point is. It doesnt really silence things, just makes it not sound as much like a gun shot.
My ears are pretty bad, from a lot of years of both protected and unprotected gun fire, and exposure to loud heavy equipment and heavy construction environments. While I can shoot both my AR's and suppressed handguns without wearing ear plugs, it only goes so far before I notice my ears starting to hurt. Its not like they were unprotected, but after about 50 rounds of 5.56, and about double that of 9mm, my ears are "sore", for lack of a better term, and sound in general at the end of the day, is "dull".