I think it was recognized that they needed something bigger than the 0.50 cal HMG. The USN held a "Joint Fighter Convention" in 1944 and the moderator told the attending aircraft companies that they should provide room in their designs for the oncoming T17E3 .60 cal Aircraft MG. It was pretty impressive gun, had a very high muzzle velocity but never made it to use. Some large production contracts were canceled.
This link will provide you with more than you probably want to know about the T17E3 .60 Cal MG. Just scroll down on the posts.
https://www.milsurps.com/showthread.php?t=32509