Inbefore the "paladins of realism" jump on me, small disclaimer: I request this option to be off by default and only enabled on "Recruit" difficulty and by players/server hosters wanting it.

Practically all modern FPS have some form of feedback, when the shot player has fired has reached the target. For example:
Battlefield: http://i.stack.imgur.com/7RoSJ.jpg
Call of Duty: http://media.pcgamer.com/files/2012/08/call-of-duty-hit-sound.jpg (+sound)
Crysis: (no picture, hits are indicated by sound)

The reason it is present is two-fold:

• provide "positive reinforcement" for learning - hitting a target from far away and hearing that sound is pleasant :)
• actually ease the learning for new players

While this option is not for hardcore play, it will certainly help both in new players onboarding and in veteran's shooting practice.

So, my suggestion:

• Add a "hit indicator" on the HUD, showing when the player shot has more-or-less significantly damaged a vehicle or a person, i.e.:
  • Hitting a person with rifle - yes (pun intended)
  • Shooting a tank with RPG - yes
  • Shooting a tank with rifle - no
  • Shooting an empty vehicle with tank gun - yes
• Add a corresponding difficulty setting, enabled only on recruit by default