Fiction
Springfield is the headmaster of Seishou Academy.
Since in new chapters of Upotte, and for quite some time in Upotte Nano, there are shown to be male teachers who are also guns (M3 Grease Gun and Suomi 26 more prominently, even if their faces are still to be seen as an in-joke by Tennouji), it is quite clear that both he and Garand sensei are actual guns
Fact
What we assume Springfield, as in Springfield Armory is the major producer and storer of military rifles to the Union Army of the American Civil War up to 20th Century United States.
U.S. Army Use
The Springfield 1903 was the first Unites States high powered rifle that had a magazine of 5 rounds. But it still only enough were made for the border guards and what was considered the United States military. The troubles began when 1917 happened. Even though the United States entered the Great War during the endgame after three years, the military still did not have enough 1903 rifles for the troops. Embarrassing enough there were more unofficial wartime produced M1917's ready for service than 1903's because three other armories including Springfield were currently making M1917 at the same time.
When World War Two happened, the United States soon realized that their newest 5 year old M1 Garand Rifle did not have accessory options. After the lessons of WW1 the use of snipers and of trench clearing rifle grenades was not to be ignored .So the army kickstarted the 1903 again, gave it a four groove rifle twist, and renamed it the M1903A3 as an Army designated sniper/rifle grenadier rifle. What happened was all the rifles were made at Remington and other Remington share owned groups with a serial number that started at 3,000,000. The plan that the army never used for was that the M1903 was never used by front line infantry as a reserve rifle. Eventually there were accessories for the M1 garand, and those accessories transferred over to the 1903A3. Springfield however became a primary producer of the M1 Garand rifle.
The 1944 1903A4 basically had a 2x fixed magnification scope mount with the ironsights removed for cheaper production.
U.S. Marines Use
The first stages of the Guadalcanal Campaign in the Pacific; the first Marine landing forces used the 1903 Springfield as a service rifle. Because Marines just being Marines, underfunded and unknowing or it was because the Marines believed that the 1903 was still superior to the untested M1 Garand. However the 1903A3, along with other improved WW1 weapons that were handed down later were a lot cheaper than the later war production variants of the United States small arms arsenal.
Makin Island/Atoll Marine raid in part of the 1942 offensive was an operation of tactical victory of killing Japanese troops at the Garrison of a ratio of at least 4:1, The Raider's only weapon the 1903A3 Springfield. It would be the last battles that the 1903 Springfield was used in normal service until replaced by the M1 Garand in 1943 for the Marines.
Performance
In modern times what we assume now is that the Springfield 1903 was sniper rifle. We can also say that the M1917 would have been a better sniper rifle with a barrel 2 inch longer and in 5 groove instead of 2 groove (Pre 1903A3). The Springfield did it's job, making full use of the .30-06. Although half a barrel length fixed 8x magnifying telescope-scope is mountable on the rifle, it still allowed the US sniper to pick off high value targets at beyond normal sight ranges of 600 meters or more.
The M1903A4 later on was a war production model, but rapidly got replaced by the M1C Sniper rifle. However It was 2 grooved barrel, instead of 4 grooved.