Under the rule slides, uppers, and other housing would be considered firearms and would require serial numbers. However, modular firearms such as AR-15 variants, if sold as complete firearms, would not be considered two separate firearms.

Does an AR upper have a serial number?

You can find the upper serial number on the inside edge of the upper receiver. Please disassemble your upper from lower and turn upside down to locate the serial number.

Does an AR uppers require FFL?

It is not considered a firearm. ... There is also no FFL (Federal Firearms License) required to purchase an AR15 upper online. So you can buy as many as you'd like. You can also purchase complete uppers without an FFL license OR a background check.