in the 'early days' of cartridge firearms most countries were arming their troops with big and slow cartridges like the 45-70 and the 45-577 martin henry like you see in the movie Zulu.
Anyways, the brits were early adopters of medium caliber rifles, the 303 enfield bieng this. however, the first loadings were blackpowder, then they swithed to cordite as that technology became available, and finally to smokeless powder. The original blackpowder loads didn't develope velocties over 2000 feet per second so they were basically not delivering the massive wounds we see in modern 30 caliber firearms. (note how history repeats, we cut the M4 down so far the 5.56 performs poorly because it falls below a velocity threshold and doesn't deliver the lethality a small high velocity cartridge is expected)
So, while the 303 was fine for shooting 'civilized' enemies (i.e. French, Germans, Italians, Spaniards, etc) who had enough sense to stop fighting when shot and go get medical attention, when faced with 'barbarians' the blackpowder 303 just wasn't dropping them.
The 'field expedient' solution was to take your knife and cut off the tip of the bullet. The original thought was to make it flat and wide, thinking it would work like a hammer. The real reason this technique worked is because it exposed the lead core. Now rather than a FMJ, the round being fired was a semi-jacketed lead softpoint, which as any hunter can tell you is going to expand a lot better.
Note, while it was frowned upon to use these expanding bullets on 'fellow civilized nations' it was viewed as totally fine for shooting people who had darker skin, as they were 'savages'