I just came across some brochures from US Ordnance advertising QCB with single breech lock. I don't have depot-level knowledge of M2 but fired few thousand rounds from them ... and still can't find a way how this is possible. Since the barrel is fixed the only possibility how to adjust headspace is the breech lock ... and with all tolerances in the mechanism ... there must be a catch... Any ideas?
From their website:
The U.S. Ordnance M2A2 machine gun is an air-cooled, belt-fed machine gun that fires from a closed bolt and operates on the short recoil principle with fixed headspace and timing