British Army coax guns and ammo were a mess. First you have the Vickers in .303 inch, then they chose the Besa in 7.9mm as a replacement, and finally they acquired the Browning in .30 US. Three different weapons using three non-interchangeable types of ammo. At the same time the RAF were using the Browning in .303. The RAF version is interesting because the gun design was modified so that it fired from an open bolt, thereby avoiding the the problem of cook-offs caused by overheating the breech. This would have been useful to have anyway.The neat trick is that the Sherman coax used the same mounting points as the ground M1919 .30cal and M2 .50cal.
If your tank crew wanted a .50cal coax, you needed to acquire a .50 from somewhere (as simple as the AA pintle if you're a US tank) and then pull out the M1919 coax. The whole swap took about as long as putting either M1919 or M2 onto a tripod.
With the benefit of hindsight they could have avoided all this by adopting the Browning in open-bolt form but with a much heavier barrel and a reduced rate of fire. Then when US AFVs came along, it would be a simple matter to swap out the .30 Browning for the .303 version.