Mike 1967: Apologies: as per http://www.thegunzone.com/dum-dum.html I too used the term dum-dum as slang to mean any expanding jacketed projectile. But I shall not dare to do so again.
Englishman-CA: a rear sight notch to the left, as on my MLE Mk I, if erroneously fitted to the RIC carbine would indeed bring the point of impact to the left. See How to adjust an iron sight - Wikiversity