And a google search for Utah fires target shooters turns up About 1,680,000 results (0.30 seconds) so, true or not, it's out there, and it's multiplying...
Funny that doing searches on youtube.com, not 1 video I see throughout the years of the millions of youtube videos show no gunfire starting brush fires or claim that has happened.
The keywords that I used are in hyperlinks to youtube pages below.
http://www.youtube.com/results?search_query=shooting+starts+forest+fires&page=2
http://www.youtube.com/results?search_query=gunfire+start+forest+fires&oq=&aq=&aqi=&aql=&gs_l=
Although, some military guys are shooting the Ma-Deuce say they don’t shoot randomly so not to cause a possible fire.