Which items on a personal record will prohibit you from purchasing a gun? E.g. use of marijuana? DUI, Etc?

Persons who would be prohibited from purchasing a firearm as a result of data obtained from the NICS background check include:

1. Convicted felons and people under indictment for a felony
2. Fugitives from justice
3. Unlawful drug users or drug addicts
4. Individuals who have been involuntarily committed to a mental institution or determined to be mentally incompetent
5. Illegal aliens and legal aliens admitted under a non-immigrant visa
6. Individuals who have been dishonorably discharged from the military
7. Persons who have renounced their American citizenship
8. Persons subject to certain domestic violence restraining orders
9. Persons convicted of misdemeanor crimes of domestic violence

An NICS check is required for purchase of a long gun (rifle or shotgun), handgun, or both.

When a licensed dealer performs a background NICS check on an applicant, he can discover if the applicant was dishonorably discharged from the Armed Forces; is an unlawful users of or addicted to a controlled substance; has been adjudicated as a mental defective or been committed to a mental institution; or is an illegal or unlawful alien.

So, only if you have been caught with marijuana or busted on a DUI will that show up. Actually, DUIs will not prohibit you from getting approval to get a gun unless the DUI charge was criminal. 1st and 2nd offenses are usually misdemeanors.

When you apply for a gun license, the store running a check doesnt know what your prior offenses are, only whether you can apply for a permit to purchase a handgun. Thats it.