It's a tray of 154 rds of 9mm.  Stacks 5 high, in two columns in your standard ammo can.  I've heard them referred to as .50 caliber cans, but mine's marked for M855.  It's 5.5"x11" approximately anyway.

• Watch out for "elephant's foot" causing a snug fit.  Either turn up the elephant's foot compensation distance, or expect to do a lot of deburring.

• I printed in PLA, so far so good.  I expect problems when summer comes, so maybe try printing in something more heat-resistant.

• The tray feet are in weird spots, and the finger hole is off-center, I know.

• Bridging might be a problem for you, but it was no problem for me with a MK4S and PLA.