I was looking at the picture I cobbled together on page 2 of this thread where I had a VRF14 modified to work like Knight's Armament Co. Masterkey, but with twice capacity. Instead of a standard 14" barrel I shortened the forend by one rail slot and cut the barrel down to 10" like the Masterkey. Then I measured how many pixels the 14" barrel on the original pic was compared to the 16" M4 barrel. The 14" barrel was a hair longer, so I cut that part of the picture down to 85% on the 10" barrel and reassembled it. When I was done, I realized the 12 gauge mag was now the same size as the 5.56mm mag. They're a lot closer in size IRL than you may think. The 3" 12 gauge mag is only about 1/4" longer, so if it was only for 2 3/4" shells instead, it would be roughly the same length as the 5.56/.223 mag. So I probably should have only cut the VRF14 down 90% instead of 85% to get the scale right. But this is still a whole lot closer to scale than the pic in post #18 where I was going by the size of the trigger guards, etc.
Here's how it would compare to the Masterkey that Knight's used to make. And here's a picture of a Ciener Ultimate Over/Under developed in 1989 by Jonathan Ciener as a competitor to the KAC Masterkey. It wasn't just a Hollywood movie gun. The Ciener Ultimate has allegedly been used by Delta Force, and the design's unique mounting system is mentioned in Mark Bowden's Black Hawk Down: A Story of Modern War. Paul Howe was using a "CAR-15, a black futuristic-looking weapon with a pump-action shotgun attached to the bayonet lug in front." The Ciener design was the only underbarrel shotgun to use the host weapon's bayonet lug as an attachment point at that time.
https://en.wikipedia.org/wiki/Ciener_Ultimate_Over%2FUnder