Would normally mean a hand made hollow point-like bullet. Made by filing a groove, drilling a hole or any other modification to a normal bullet to cause it to mushroom or shatter on impact with a body. The term comes from the British colonization of India, they had an arsenal at Dum Dum where they were "invented". They were eventually made "illegal" by most armies and in some armies merely possessing dum dums was a hanging offense.
I suspect in this case it may be a vernacular term for hollow points as opposed to the hand made variety.