XM914 50 mm.

This is coolbert:

AND WHAT EXACTLY IS MEDIUM CALIBER AMMUNITION?

See the previous blog entry and herewith the answer.

"NG [Northrup Grumman] manufactures a range of shells, usually fired by chain gun cannon, in the calibers 25mm, 30mm, 40mm, & 50mm.

25/30/40 mm rounds all I have heard of. 50 mm I have not ever heard of."

As to the 50 mm medium caliber round:

"AUSA 2019: Debut of XM913 50mm Bushmaster Chain Gun for U.S. Army's [NGCV] Next Generation Combat Vehicle"

See the You Tube video:

See also the wiki entry the U.S. Army NGCV vehicle. 

Chain gun a proven winner. Combat tested very successfully. Not a new technology to be frank? But in this instance with considerable more lethality.

coolbert.