Where from? Who did they steal from? ENLIGHTEN ME!!!
In US as well as Mexico, most of the guns used for crimes are purchased at gun shows where private party sales technically require the seller to check the buyers ID to make sure they're from the same state and are complying by the rules (i.e. Texas, Wisconsin, etc - the Texas/Wisconsin/etc private party seller is obliged to check the ID of the seller to make sure they're from the same state). However, about half of sellers won't check ID and will just take the cash and off you go with your full auto. Those guns are then resold to gangs and individuals in states that have very strict gun laws like NY, Illinois, California, etc. where there's lots of shootings. However, in the states that have lax gun laws, i.e. Alaska, Wisconsin, Indiana, Texas, etc the amount of shootings/gun deaths in generally lower than places like New York, California, Illinois, etc.
Gun thefts aren't as common but they do occur.
Go get the AK in Poland.
Rather easy to obtain a gun in Poland, legally and illegally. Ex-military and cops are able to keep their guns for life. If you want a license for a gun you can't have a record and have to take certain tests - some of which may include medical/psychological depending (i.e. drug test, making sure you're not crazy, etc.). Bobby pistols are sold openly in malls and can easily be converted into a real gun. Also, if you know where to look in certain targi you can buy old soviet guns. The process is actually pretty similar to the US. Only difference is in Poland you actually have to take a medical/psychological test whereas in the US you don't have to take such a test but if you ever were in a mental institution or have diagnoses of mental instability or drug use its generally enough to bar you from buying a weapon.
A reporter from a rather left leaning outlet wanted to prove how 'easy' it was to buy a firearm. Well, he failed because a very old domestic misdemeanor popped up on his record from when him and his wife got in an argument and the cops were called. That was enough to disqualify him.