The M-14/M1A was a battle rifle before it became an M21 sniper rifle @darthnava. It's primary use was to be carried by Grunts into combat and used with iron sights.

It was what the M-16 replaced and there was a good many years where they were both issued as primaries at the same time by different services in the U.S. I even managed to get my hands on one a few times when I was stationed in Germany in the 80's, even though they weren't standard issue well before then. Helped us do better on a few training exercises against vehicle targets.