making homemade Nerf darts

I'll second all of what Ali said, but I'd also like to add that not all games these days even allow homemade darts due to safety concerns. The worst-case here would be a felt tip falling off and exposing the metal washer underneath.

For this reason some darts were made without a washer at all - and with either a solid lump of hotglue serving as the weighted tip or with a small BB in the bottom of that hole for weight (which guide Ali linked might already say? There's a follow-up post saying that it was updated to include metal-free dart tips but I can't find where it describes how to make them.)

OP, I'd recommend reconsidering whether you want a hard tip. If you're running into issues with people not feeling and not taking hits, improving the velocity of your blaster will solve that while also providing greater range a difficulty-to-dodge. (If your problem is people "not feeling" and not taking hits - then that's not something that any modification can solve short of something actually dangerous.)

If you do have a good reason for wanting a harder-than-normal tip, then 3d printed tips are an option. TPU is available in a variety of hardnesses and differing infill and tip geometry should theoretically allow you to design a tip with whatever characteristics you want. I wouldn't recommend this - most people don't other with homemade darts at all any more and there's good reasons for that - but it is an option if you just want homemade darts.