In the old days (so I’ve read, I wasn’t really there- plenty of folks on here were though) the long version AR with a fixed stock was a rifle and the shorter one with a collapsible stock was a carbine. Hence the birth of the terminology. Things have changed, but even then you could have run a carbine stock and buffer setup on a rifle platform. We’ve learned to mix and match over the years and when you’re talking carbine or rifle there’s two halves you’re talking about. The gas system, and the recoil seystem/stock. There’s also a bunch of other stuff that’s been thrown in the mix over time. Mid length gas, different stock and buffer setups. As long as you understand them and plan accordingly, you can make a lot of combinations work just fine together. I have noticed they don’t make a mid-length buffer tube....interesting, no? Not really, two separate halves with two different functions - similar names due to history.