I know that the Mega lower is marked as "multi" as it will take all AR parts. I have mine set up for a .223 right now for coyotes. Can it be set up for lets say a .243 or a .308? It looks like the clip size is limited.
There's a lot of information on this, but basically you can use whatever will fit inside of your mag well. For an AR-15 some of the commonish options are:
7.62x39
5.45x39
5.56x45/.223
6.5 Grendel
6.8SPC
300 Blackout
.50 Beowolf
.450 Bushmaster
There are many others, some of them require new bolts, but thats a really minor deal. Obviously you'd need a new barrel too. If you wanted multiple calibers you'd usually be best having a dedicated upper for each one and just pin it to your lower and be good to go again.