Good shooter for an older PC?

Like the title says....I have an older PC so I can't play the newer shooters. I'd like to play something like Battlefield that is not so resource heavy. Any ideas?

Depend on what you prefer: single player or multiplayer.

For example I still could play Return to Castle Wolfenstein (even it's very old game).

Other games:
Medal of Honor: Frontline
S.T.A.L.K.E.R.: Shadow of Chernobyl
Borderlands 2
Arma 2
BioShock
Left 4 Dead 2
Rainbow Six Siege

From old fashion games i would like even they would bring alive now:
Iron Grip: Warlord
Medal of Honor series like "Allied assault" and also it's expansions like Spearhead or Breakthrough; Pacific; i think Allied assault multiplayer it still running with some cool modded servers
A little horror and exploration and puzzle game like Penumbra series Overture, Black Plague, Requiem.
Of course how without cheeki breeki S.T.A.L.K.E.R. series.
Also the Deus Ex: Human Revolution.

half life 1 and 2
portal
cry of fear
resident evil 4

The original doom is one of the top hands down shooters of all time.

I don't know if you could classify it as good but redneck rampage is worth it for the sound track alone.

Call of Duty 2 and 4 , CS GO , Battelfield , Sniper Ghost Warrior 1 - 3

I mean, you havent listed your PC specs. How are we supposed to know what games your PC can and can't play?

I recommend you to try ARMA 2, smooth and beautiful game