*sigh* it's not a machinegun because the ATF says so. That's it.
There was a letter last year or so, which stated (it cannot be stressed enough that these letters are non-binding) that a single trigger could fire five AR barrels (if such a design were made), or five triggers could each fire a single barrel in succession. 'One bullet per barrel per action of the trigger' seems to be the default position these days, unless your design seems too controllable in rapid fire (or the Bureau hates you). In which case they'll basically lie to prevent your design from coming to market. This is basically the only way cheap SxS's can be tolerated, since even single trigger models are notorious for doubling.
TCB