Over 100 million of the weapons have been sold across the globe and , and has become synonymous with civil wars, guerilla fighters, as well as terrorists.
The rifle brought Kalashnikov fame and a string of Russian awards as well as the Order of Lenin.
But he never made any money out of it and once said he would " have been better off designing a lawnmower."
Kalashnikov,who first started to design the rifle in the Second World War, previously refused to accept responsibility for those killed by it.
But he wrote to the Patriarch of the Russian church said the designer felt a degree of guilt and " spiritual pain" towards the end of his life for inventing the killing machine.
The letter published by Russian newspaper Izvestia quotes his daughter, Elena, saying she believes a priest helped her father compose the letter, which said:" My spiritual pain is unbearable."
He asks: "I keep having the same unsolved question: if my rifle claimed people's lives, then can it be that I... a Christian and an Orthodox believer, was to blame for their deaths?
"The longer I live "the more this question drills itself into my brain and the more I wonder why the Lord allowed man to have the devilish desires of envy, greed and aggression".
The letter is typed on Kalashnikov's own personal notepaper and with the wavering signature describing himself as "a slave of God, the designer Mikhail Kalashnikov".
Apology: Kalashnikov feared he was to blame for millions of deaths
The AK-47, named after the year it was completed, is one of the world's most familiar, feared and used weapons.
Its comparative simplicity made it cheap to manufacture, as well as reliable and easy to maintain.
He died on 23 December at the age of 94 after being admitted to hospital a month earlier with internal bleeding.