I left my hidden location with Abraham's gun, alarming Nightwielder and his companions, and plays the part of a store employee, telling Diamond that he fixed a non-existent defect on the gun, and replacing it on the wall in the empty spot where the gauss gun was. Nightwielder asks David to explain the specifications on Abraham's gun and another one, which he does easily.

Nightwielder buys Abraham's gun and orders more like it, and asks for forensics tools which Diamond stocks just for him. David reads the specifications for one of the forensics kits to figure out if it has an ultraviolet scanner since he thinks that Nightwielder is vulnerable to UV radiation, and finds that it does, then contrives to test this hypothesis by turning on the scanner and pointing it at Nightwielder, knowing that this could mean his death.

-Summary

http://coppermind.net/wiki/Steelheart/Summary#Chapter_1