How can someone's clothing show that they have fired a gun?

We can tell someone’s clothing show that they have fired a gun when the burning gunpowder residue stays or remains on the clothes. If the clothes test positive for GSR gunshot residue it means that it is very close to a gun that was fired. Remember that a traces of a gunpowder residue remains on clothing or skin and on the surface after discharging a firearm is 78 hours.  

1) union forces launched a surprise attack.

2) more confederate troops arrived by train.

3) union forces retreated.