I was using a UNI insert on a XX78. It seems as though that might have been leaving some silver metal on the rail. You can see marks on all three arrows on the left side right at the rear end. i think the UNI insert might be just a hair fatter than the aluminum shaft. I replaced them with regular inserts, so we'll see if that does anything.. Although, the aluminum shaft itself also had wear marks on it at the rear end.. I DONT KNOW