Is it possible to take a shielded unit down by shooting the shield?

Shields can be killed from the front (through their shields) when using a shotgun equipped with AP Slugs or when using a sniper rifle.

From Payday Wiki:

Combining any shotgun with AP Slugs grants the weapon with the Shield-piercing capability

...

Shield units can be killed by shooting through their shields with any sniper rifle.

Additionally, there is an achievement for killing 10 Shields through their shield using the R93 sniper rifle (Maximum Penetration) and an achievement for killing 50 Shields with AP Slugs (Knock, Knock)

No, you can't kill a Shield by shooting on his shield.

(...) He is armed with a B9-S pistol and carries a large riot shield, making him invulnerable to bullets from the front.

Source : http://payday.wikia.com/wiki/Shield