Source: ABC News By VLADIMIR ISACHENKOV Associated Press In a regretful letter penned a few months before his death, Mikhail Kalashnikov, the designer of the AK-47 assault rifle, asked the head of the Russian Orthodox Church if he was to blame for the deaths of those killed by his weapon. The Russian daily Izvestia on Monday published the letter, in which Kalashnikov, who died last month at 94, told Patriarch Kirill that he kept asking himself if he was responsible. The AK-47 is the world’s most popular firearm, with an estimated 100 million spread around the world. “The pain in…