AR-15s

Sig makes a decent upper that gets rid of the buffer tube. Based on a Daewoo/AR180 style system. Brownells sells an AR180 upper conversion too which allows you do to away with the buffer tube as well.
The way I see it: it's a carbine because I want the stock. Folding is for transit, not for firing Mogadishu style with it folded. Every vehicle borne operator deploys from their vehicle with the stock set where they run it, and if they can run a 16" carbine from the cab of a hilux, then smaller doesn't seem like a huge requirement.
That said, I still built me a little CETME LC just because the collapsing stock on those is so damn cool!