This is also the reason you should not fire you (military) rifle with the flash hider, since it also makes sure the front end of your barrel does not "split" open.
I don't understand this at all. Can you clarify?
What Gryphon said. Boiled down somewhat for quick consumption:
There are 2 types of firearms, Title 1 and Title 2.
Title 1 firearms can be owned, with no special licensing, by anyone that can legally own a firearm in their area. They are rifles with a barrel length no less than 16" and overall length of 26". Shotguns must have a barrel length of 18" and have the same overall length requirement as rifles. Pistols and so on.
Title 2 firearms are basically anything that doesn't fall into title1. Machineguns, short barreled shotguns (sawed-off), Short barreled rifles (SBR), etc.
It's a long, convoluted, and nasty group of laws and rulings that dictate what's available. It seems like a minefield sometimes, particularly when building something from parts.