http://www.remington.com/library/history/firearm_models/shotguns/model_10.asp
Yes, I just bought one
Are you sure it's an auto and not a pump action, it sounds like the predecessor to the Remington Model 10.
The Remington Model 10 slide action has barrel lengths of 26" 28" 30" and 32"
I just so happen to own a Remington UMC model 10, actually it is pre model 10, they did not name this the model 10 until 1911 I believe. Mine actually has the dates 1903 and 1905 on it. But to get to your answer it stands for UNION METALLIC CARTRIDGE.
To determine the age of your Remington Model 10 shotgun with the serial number U55191, you can refer to the Remington serial number reference charts, which indicate that the Model 10 was produced from 1908 to 1929. The "U" prefix suggests it was manufactured in the 1920s. For an exact year, you may need to consult a detailed Remington production database or a firearms expert.
According to Remington, it was made in 1913.
1920
In reference to what application?
1909
1920
1910
1912