For a user of recreational marijuana in CO, Oregon, etc., is it legal to own and purchase firearms?

While I completely disagree with the federal law in this regard, be aware that it is illegal for someone to possess a gun if he or she is ‘an unlawful user of, or addicted to’ marijuana or other controlled substances. This is a highly political and gray area of the law. A possession charge will make it far more difficult to obtain a concealed carry permit and without that permit, one would open himself or herself up to very serious charges. The best move for anyone charged with simple possession of marijuana is to hire a lawyer and try to get it dropped or perhaps amended to a non-drug offense. Each case is different, and immigration / citizenship issues may also come into play, so please do talk to a lawyer if charged.

Another problem with possessing both guns and drugs is that police could suspect you are not just using drugs, but also selling drugs. From the perspective of a police officer, when they see someone who has both guns and drugs, they automatically start thinking you might be dealing drugs. If you are charged with selling drugs, the potential penalties are extremely stiff. You might be found not guilty at a trial, but you could have to pay a high bond or sit in jail awaiting trial.