You always have to get them engraved. Just for the ATF compliance. Once they're registered as SBR lowers, you can use them for any length upper. Long or short.....doesn't matter. And even though you have to state a barrel/overall length, you don't have to worry about it once it's registered. You can run even shorter uppers on it if you want. They just want their 200 bucks for the stamp, and make you jump through the hoops. After that.....it's game on.