Is it legal to have a medical marijuana prescription and purchase a firearm? No.
Look at the BATFE's 4473 form, called the Yellow Sheet, which you must fill out when purchasing a firearm through a dealer.
Look at question 12 e. It asks a purchaser: “Are you an unlawful user of, or addicted to, marijuana, or any depressant, stimulant, or narcotic drug, or any other controlled substance?”
Remember, this is a federal form, so it’s asking if the purchaser is a user to a (federally deemed) unlawful substance. Federal law still considers the use marijuana illegal. So, a legal Colorado user of marijuana could not answer 12 e. with a “No”. And if you answer “Yes”, you can’t buy a firearm from a dealer.
Also, the 1968 Gun Control Act says (TITLE 18 > PART I > CHAPTER 44 > § 922) click here for a link:
(d) It shall be unlawful for any person to sell or otherwise dispose of any firearm or ammunition to any person knowing or having reasonable cause to believe that such person—
(3) is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802));
Can someone who has a medical marijuana prescription possess a firearm? No.
18 U.S.C. § 922 (g)(3) and 21 U.S.C. § 802 both prohibit anyone who is a user or is addicted to any controlled substance, including marijuana, from possessing, owning, purchasing, receiving, or having within his custody any firearms.
Can someone who has a medical marijuana prescription get a concealed handgun permit? No.
From Colorado Revised Statutes, 18-12-203
Criteria for obtaining a permit
(f) Is not an unlawful user of or addicted to a controlled substance as defined in section 18-18-102 (5). Whether an applicant is an unlawful user of or addicted to a controlled substance shall be determined as provided in federal law and regulations.
So, on every CCW application, you'll see this question:
"Are you an unlawful user of, or addicted to, marijuana, or any depressant, stimulant, or narcotic drug, or any other controlled substance?"
So, unless federal law is changed, application for a permit usually is denied, since federal law still considers the use of marijuana unlawful