Fox, you beat me to it with a better mod. Good one on the plaster! Even if the plastic shell plate cracks, nothing pours out. Sand is also great. I basically stole tiny rocks from a childs playground and ran home then filled mind. I taped all the sides in case if I fell, the shell would cracking causing rocks to pour.
These mods are a plus because of the weight. Especially on a PC, even with mags and hollow SAPI plates, the shoulder pads feel like it's not really hugging. After putting these weight plates in, they're perfect.
Especially for impressions, they're a plus. Someone lifting your second line would be amazed. With rocks, they weight abour 7 pounds.
Who cares if they're "heavy" ? Don't be a pussy. It's a great workout on the field.