If you want a job which involves shooting guns all day, then you are looking at being self employed as a firearms instructor. And starving most of the time.
If you're looking for a career in a gun carrying field, then you have several choices to make BEFORE you do anything. You need to decide whether you are going to be a professional or skilled labor. IF you're going to be professional then you need to decide what degree you need and what university to go to.
Skilled labor jobs are things like armed security guard, armored car employee, private investigator, gun smith, gun shop owner or employee, etc. These jobs require little or no education and entry level openings usually require no experience. Some jobs require training and certification but this can usually be obtained at the local community college.
Professional jobs are things like law enforcement (corrections, probation or LEO), Attorney (FBI agent's are usually atty's too), CSI, etc. These jobs usually require at least a bachelors degree or more. Entry level requires a degree but often no experience.
For my money, right now I'd be looking at a field other than exclusively crime scene investigations. Most city/state government budgets are going to require a reduction in payroll and layoffs due to the economic situation.
A better bet would be to look at the possible future employment requirements 5 - 10 years from now. Jobs in environmental health, energy, and other emerging fields are a better bet than "yesterday jobs" of nursing, CSI, & law.
As for the best prospect: become a doctor.