I found this via a search.... don't know if they would accept citizens or not (if not... just another special perk for the police... ).
I also found these on the NRA website... perhaps you can contact them for suggestions as well? Good luck with it.
Guam Outdoor Shooting Range - Tamuning, GU
PMB 261, 1270 N. Marine Corps Dr
Tamuning , GU 96913
Business Type : Ranges
Phone: (671) 828-1153
Islanders Sportsmans Club , Yigo , GU
P.O. Box 4424 AAFB BR
Yigo , GU 96929
View Range Map
Phone671) 366-2034 Facilities Include: Trap, Skeet, Airgun, Archery
Range Access: Private