>>63986387
Alright, here we go.

6.5 grendel
>7.62x39 (ak47 cartridge) necked down to 6.5mm (.264). A marvelously efficient cartridge - this is what proper cartridge design looks like
6mm arc
>6.5 grendel necked down to 6mm (.243). A pretty popular long-ish range cartridge for the ar15
338ARC
>same case as the above 2 cartridges, necked up to 8.6mm (.338). Its large bore and relatively small powder charge makes it a good SBR cartridge
300 blackout
>5.56 necked up to 7.62mm, and with a shorter case. Again, good SBR cartridge.

So the first 3 have a larger case diameter than 300 blackout. This means they can’t double stack in a polymer magazine and still fit in the ar15’s magwell, hence the need to make a non-standard lower receiver with a wider magwell to accommodate wider polymer magazines (though metal mags with thinner walls will still work in a normal AR)

Picrel. 6mm ARC is the parent case of 338ARC and so you can see why one needs new mags and a new receiver. It’s just too much bigger than 300 blackout (300 AAC).