Why, when they can be bought?
They’re called hollow points, or soft points, and are (pandemic scarcity aside) normally a lot more common as a reloading component than FMJ…particularly in non-military calibers.
The most common way of making a “homemade dum-dum” is to drill a hole in the tip…except if you don’t get that hole perfectly concentric, the bullet is going to have an eccentricity and will therefore be inaccurate—probably wildly inaccurate, at that. Don’t know about you guys, but is much rather have an accurate FMJ than an inaccurate round of any other type, any day.
Additionally, there’s a lot more to making a JHP perform when it hits than just a cavity in the nose; look, and I mean really LOOK at a modern hollow point know for good performance, and you’ll see things like skives (perforations) in the jacket, as well as contouring inside the cavity.
No…I think it’s a “dum-dum” idea, myself.