Former federal prosecutor David Weinstein's analysis on State witness FDLE crime analyst Amy Siewert:

The important thing about gun shot residue is that it can show where an item was in relation to where the gun was when it was fired.

In this case, it is a semi-automatic gun. Powder residue will be emitted from the barrel/muzzle and the slide where the empty casing ejected.

Humidity, moisture and what the clothing is made of will impact all of this.
For both sides, is the gun shot residue consistent with a struggle or a point and shoot.

For both sides, is the gun shot residue is consistent with a struggle or a point and shoot.

In this case the FDLE analyst would testify that physical effects demonstrate a "contact shot" that the barrel of the gun was against Trayvon Martin's clothing.