And this is exactly why the rules regarding handgun barrels and pieces of such exist. When the governments "firearms experts" were making up all this foolishness, there was a huge concern over short barreled handguns, the ones they liked to refer to as Saturday Night Specials.Old pistol barrels are abundant, and are free (or extremely cheap) when they are replaced and discarded.
Because they considered these firearms to be more dangerous than toxic waste, they wanted to be sure that they removed all means of creating more. So the rules for pistol barrels were created, the fact that the same rules don't exist for rifle and shotgun barrels was just an oversight on their part. If the whole gun control program did not go off the rails I am sure they would have gotten around to "fixing" that too.
SandRoad you are probably referencing on-line material and it is far from complete and that is what is confusing you.
Think of these scenarios: You have a plumber or gasfitter that has lenghts of cast iron pipe on their truck. You could easily find a round that would fit inside one of those pipes, so are they considered barrels? How about if they cut a 3" section off of one of those pipes, have they created a short prohibited pistol barrel?
What about a hardware store that carries 2-3" pipe nipples that you could fit a round inside, are they prohibited short pistol barrels?
How about every metal shop across this country. If you went through all their tubing you will easily find several diameters that are capable of "chambering" ammo. What about all the pieces in their cutoff bins are the short ones prohibited pistol barrels?
In my shop I have 12Ga barrel blanks and heavy wall seamless tubing that look identical. Which one is a barrel and which one is just tubing? Neither one has any identifying features that stand out but both are capable of being made into a barrel. Is it simply the fact that some of them were sold to me as barrel blanks?
There has to be some way of identifying a barrel as such and that is where the definition of "being capable of chambering and discharging ammunition" comes from. This is the definition that exists in the laws and it applies only when used in the context of firearms ie: it's not a barrel if you can put a round inside a tube and discharge it with a finish nail and a hammer. The barrel has to be attached in some way to a frame or receiver that is considered a firearm. The means test will be is it a functioning critical part of a firearm.
So in the case of a chamber adapter it is not in any way attached to a frame or receiver and the firearm functions without out it. So it is nothing more than a metal tube and barrel rules don't apply.
Clear as muddy water now?