I am a big fan of building AR-15 rifles, to me they are legos for men. You can easily put together an AR 15 receiver in about 30 minutes if you have the parts and a good tutorial. I have a couple I have don on this site.
Of course the problem is the receiver, they are much harder for a person to make. Below I have a set of blueprints to allow you to carefully machine out your own lower from a block of aluminum. This is, at the current time, completely legal. You can build your own recievers if you aren’t building them to sell and are not prohibited from owning a firearm.
The issue is when you let someone else do it. I have heard of machinists getting in trouble because they let someone “build their own” receiver by pressing the start button on a CNC machine that the machinist set up.
Also in the event you are interested, all of these gun documents I am posting can be found on the disk I have listed to your left. Additionally to the hundreds of documents I have posted, I have hundreds of gun patents (mostly suppressors) as well as class powerpoints and targets. all for $5.