[ceserthen]

A better idea I had a while back (but never posted) was a more of a suit than a shield.

Plasma shield
all plasma bullets decrease by 1/2 their original damage. If a regular bullet hits you though it would damage you by twice the original damage

Reinforced armor
Decreases all regular bullets by 1/2 their original damage. If a plasma bullet hits you it does twice the original damage.

This system would not run on ammo but you would still get damaged.