it is 3006 cal.
The Model 1903A3 rifle is base upon the Mauser 98 action but they are not exactly the same.
I checked www.oldguns.net and they show the manufacture date of Remington 1903A3 serial no. 3851292 as 1942.
With the serial number that you have supplied,your Smith-corona model 1903A3 was made in the year 1943.these rifles were only made in the years 1943-1944.If your rifle has a letter C prefix to the serial number,then this indicates that remington also made a model 1903A3 with the same serial number. These were the only 2 manufactures of the 1903A3 rifles.
made 1942
February 1943
The Springfield 1903A3 rifle with serial number 833946 was manufactured in 1943. The production of the 1903A3 variant began in 1942 and continued through 1944, primarily during World War II. This particular serial number places it in the middle of that production timeframe.
August 1943
The Remington 1903A3 with serial number 3941921 was manufactured in 1943. The 1903A3 was a variant of the M1903 rifle used during World War II, and Remington produced these rifles primarily between 1942 and 1944. Serial numbers in that range indicate production during the wartime period.
1944
September 1943
July 1943
August 1943