It varies with where you are. Some places don't care what parts are in the gun, as long as it does not fire full auto. Some places have laws about "machine gun parts". In those places it is possible to have "illegal" parts in your semi auto only rifle.
The ATF at one time declared the auto sear, (all by itself, no other parts or gun needed) to be legally, a machine gun.
Amazing what the boffins in govt service can do, isn't it?
Safest thing, no M16 parts in your AR. M16 parts in your AR, but NOT capable of full auto ought not to be any concern of anyone, but in some places, it is, due to state (and maybe local) laws.
I know one state that the way the law is written, it not only prohibits private ownership of machineguns, it also prohibits ownership of machine gun parts. Technically, even unservicable parts used as a paperweight could be illegal there.
Most folks don't know, and can't tell the difference in the AR parts, but rest assured, if you are ever in a situation where the legal system gets interested in what is inside your AR, they will bring in an expert who can tell.
All else being equal (and it almost never is) bigger bullets tend to work better.