Could American 14-inch ammunition support the King George V class?

Could the Royal Navy have relied on U.S. supplies of 14-inch shells?

No. British and American 14-inch guns and shells were not designed for ammunition commonality.

U.S. 14-inch ammunition was not a ready British supply source. Differences in gun and shell design meant the rounds lacked the required commonality. Adopting another calibre increased the Royal Navy's logistical burden even when an ally used the same nominal bore size.