Totally legal to own unless your state has outlawed them. They become illegal when you replace your AR15 parts with these parts PLUS a DIAS (Drop in auto sear). To possess a DIAS legally it must have been manufactures prior to the 1986 NFA. To buy a Pre'86 DIAS, you have to make the purchase through a Class III FFL retailor, ~$20,000 plus $200- for the NFA Treasurer stamp which you get after you have passed the six or so month background check.
Once installed in your AR15 along with the select fire group advertised in Nemo's post, your AR15 is now classified as a machine gun.
Clear as mud huh?
Now, if you want to spend 10-years in a federal prison, because you don't want to drop $20K for a legal DIAS, you can make one which is very easy to do. There are blue prints all over the interwize. I have heard of some folks who have purchased the legal to own fire group for the future. And if the shit hits the fan, build their own DIAS at that time. The feds will be too busy with other distractions to come looking for you.
One last comment: Not every AR15 can be converted into a select fire rifle with the aforementioned fire group and DIAS. Some like the Colt AR15will need a small bit of dremeling and a tweak here and there. While others like the Bushmaster are pretty much swap out drop in and go.