You could register as a manufacturer and make your own; or you could a dealer's license and play with post-86 dealer samples (you have to get a letter from a LE agency requesting a demo)
Both options are expensive and a lot of paperwork. When you stop paying for your license, you will have to sell or destroy all of your post-86 goodies.
If you buy a transferable, pre-86 select fire rifle it's your's forever, but since there is a very limited supply prices have gone way up. You're mostly paying for the transferable status, not the firearm itself.
Once I saw guys who really knew how to shoot semi-auto, full auto from a lightweight rifle or subgun holds less mystique. You'll notice in the video the guy usually shoots short bursts and his cyclic rate was pretty low. A really good shooter on semi-auto could probably hold his own in that contest. I cannot deny it looks really fun!