Mikhail Kalashnikov, the designer of the AK-47 rifle, wrote a letter to the Russian Orthodox Church before his death expressing concern that the weapon had killed so many people, Izvestia newspaper has reported.
"My spiritual pain is unbearable," he wrote in the letter to the head of the church, Patriarch Kirill.
"One question keeps repeating itself; as my assault rifle killed so many people, does that mean that I, Mikhail Kalashnikov ... an Orthodox Christian, am responsible for people's deaths, even if they were enemies?"
A spokesman for the church confirmed to Izvestia that it had received the letter and that it had been written about six months before Mr Kalashnikov's death on 23 December.
Its authenticity could not, however, be confirmed independently.
Mr Kalashnikov, who was 94 when he died, said many times that he was proud of his invention but also regretted it was used by criminals and child soldiers.
The AK-47, which is cheap to produce and easy to maintain, has killed more people than any other firearm.
Mr Kalashnikov said in 2009 that the rifle, which he created in 1947, had been intended mainly to defend his country's borders.