In an interview with the BBC, Mr Obama said it was "distressing" not to have made progress on the issue "even in the face of repeated mass killings".
He vowed to keep trying, but the BBC's North America editor Jon Sopel said the president did not sound very confident.
However, Mr Obama said race relations had improved during his presidency.
Hours after the interview, a gunman opened fire at a cinema in the US state of Louisiana, killing two people and injuring several others before shooting himself.
In a wide-ranging interview, President Obama also said:
- The UK must stay in the EU to have influence on the world stage
- He is confident the Iran nuclear deal will be passed by Congress
- Syria needs a political solution in order to defeat the Islamic State group
- He would speak "bluntly" against corruption and human rights violations in Kenya
- He would defend his advocacy of gay rights following protests in Kenya
- Despite racial tensions, the US is becoming more diverse and more tolerant