That seems to be a popular thought that probably started in WWI when US GI tried German8mm Mauser rounds in their M1903 Springfields and M1917 Enfields. Probably several GI telling war stories in the trenchs that the German rounds can be used in a pinch if we run out of ammo. It only takes one or more Sad Sacks to try. Hatcher actually studied these incidents after the war. The same story was probably told to his son who carried an M1 rifle in WWII and tried that same as it too made the popular rounds of the BS table.
The only Russianweapon system that I am aware that uses "our" munitions but we cannot use theirs is the Russian 82mm Mortar System. The Russian 82mm mortar can use the US 81mm Mortar rounds but the US 81mm mortar cannot use the Russian 82mm mortar rounds. I know the US military actually publish the firing tables for the use of US 81mm mortar rounds in the Russian 82mm mortar.
The other two tales that seems to get around is that the M165.56mm round is too small to kill. I usually tell the less informed "Then way does the US military still use the round since the mid 1960's I am pretty sure it is not to be humane. The other is the M16/AR platform does not need to be cleaned as it is self cleaning. While there is a bit of true to that statement it was a very errorous selling point in Vietnam that the rifle did not need to be cleaned. Yea, that got a lot of good Marines and GI killed. But its a story that will not die.
I had another one that an AK47 will not hit a man size target at 100 yards. I took the "gentleman's" AK and put 5 rounds the size of a fist into the chest of a man size target off hand standing. I gave the rifle back to him and said, "operator error."