A lot of soldiers use AR15s these days. Some still use the M16. This is a very generalized answer,but I hope it is useful.
the little notch just ahead of the safety notch on the side of the slide is to hold the slide open . once the slide is held open twist the barrel clockwise until the barrel has turned 1/4 turn make sure the mag is out and it should then come apart
Go to Remington.com and download an owner's manual. most modern choke tubes have some type of marking that will allow you to determine the choke without removing it from the barrel. some use a notch system while others have stars.
While there are several techniques used by gun makers, the original method was to start with a smooth bore barrel. A long screw with a single cutting tooth was inserted in the barrel, and pushed down, turning as it went. This would scratch a spiral line on the inside of the barrel. Raise the cutter a notch, repeat. Eventually, the scratch becomes a groove cut into the metal. Rotate the screw, repeat for the next groove. Some barrels had 2 grooves, some 4,5,6, etc.
they have notch teeth.
Depends on the laws of where you are. In most of the US, Yes, they are legal. Some states have specific restrictions to meet their state standards (such as California, but there are versions that are legal there. Outside of the US, laws vary greatly. Some countries do not permit private citizens to possess semi-automatic firearms.
Notch
Well a V-notch gives better accurate results when compared to rectangular notch.
You're going to need some special tools for that. This is one of those things that really isn't in the range of things you can do with hand tools in your garage. The barrel IS simply threaded into the action, but you're going to end up mangling some expensive parts if you clamp it in a vise and throw a pipe wrench on the barrel. ...and after that, then what? You can't just screw in the new barrel. There will be issues with getting the notch for the extractor proprly aligned, and then you'll have to get the chambering and head spacing right, which will require reamers you'll have to buy. You really ought to consider going to a gunsmith, this is a serious project.
make sre it's unloaded,push slide back till notch in slide lines up with slide lock-push it through to the other siide&out pull slide off frame,remove spring&rod from unger barrel & remove barrel
SLIDE THE SLIDE BACK TILL THE SLIDE RELEASE IS IN THE NOTCH IN THE SIDE PUSH RELEASE THROUGH THE FRAME AND SLIDE THE SLIDE FORWARD OFF THE FRAME THEN TURN THE BARREL NUT AND REMOVE THE RECOIL SPRING AND GUIDE THEN THE BARREL WILL SLIDE OUT OF THE SLIDE
No notch is still alive