I don't know what it is...

[kurusu]

Originally Posted by sheepherder

...???...The site I found with 3"/50 stated it was for 50 cal MGs...This gun has a 3" bore...

I was tempted to sit in one of the seats and see if I could traverse it, but I chickened out...

It's a 3"bore 50 calibers barrel length. My guess you've found the wrong site, try Wikipedia.

Edit. I just checked Wikipedia, the Gatos did indeed use them. Many other crafts too.