subsonic ammo

It took Tessa only a second to spot me. Then she darted toward me over the floor, moving with terrible, insectlike speed, and I triggered the shield spell. The runes on my staff flared green-white again, and a fine curtain of blue-green light washed out from the oak, covering the doorway.
Tessa stopped in front of it, maybe ten feet away. Then she pointed at the door with one long leg and said, in a perfectly normal human voice, “Open fire.”
The gunmen didn’t hesitate. Their silenced weapons went clickity-clack, and sparks and flashes of light bounced up from my shield where the bullets struck. Foiling them wasn’t a terrible strain. Subsonic ammunition is slow-moving by its very nature, and each individual round carried considerably less force when it struck than it would have at full speed.
“Hold,” the mantis-thing said.
The gunmen stopped firing.


Skin Game Chapter 17, Page124