.50 Beo is kinda tough, theoretically you could take something like a Robarms XCR or a Bushmaster ACR and retrofit it into .50 Beo, but since the case size is even bigger than .458 SOCOM you would likely need to open up the ejection port by milling it open. Then IIRC it uses a 7.62x39 bolt head, which are available for both the ACR and the XCR...
Something like .450 bushmaster or .458 SOCOM is a lot easier to do with the components available, a few guys here have done ACR's with .458 SOCOM or .450 bush barrels.