From some reading I've done around the issue, they had the licence to make the same 20mm Hispano cannons as the UK but for some reason had massive quality control issues producing them. I believe the USN was at one point planning on standardising on the 20mm but due to the production issues went with the .50 cal instead.
I think the P-61 (?) Black Widow was armed with 20mm though as they'd just about sorted things out by that stage.