DOOM (1993)

The Plasma Rifle is a very powerful and useful weapon, not uncommon at all to find in the later portions of the game. Given the extremely high fire rate of the Plasma Rifle, 5 seconds is more than enough to kill 5 lowly enemies. It's only available from the 2nd episode onwards, and the very earliest you can find a Plasma Rifle is in the first mission of the "The Shores of Hell" (E2M1: Deimos Anomaly) in a secret area. After reaching the blue key card, push the other button on the small block (west side). This will open a teleporter close to where the blue key card is located:


Just follow it and you'll find a Plasma Rifle with 40 rounds (or 80 if playing on "I'm too young to die"). Now it's just a matter of finding a small mob of at least 5 weak enemies (zombies/imps/lost souls) and hold the trigger till everything drops dead. Easy peasy.

Again, if you don't want to bother with finding secret areas or simply missed it, there'll be many opportunities to find a Plasma Rifle troughout episodes 2, 3 and 4.