Disclaimer: Mod at your own risk! Outback Nerf can't be held responsible for any damage to your darts/ and or blasters.
Anyways, on to the mod!
So let's start with what you'll need!
Usuable foam dart body
Streamline head/ Elite dart head
Adhesive( like glue or even sticky tape...)
Thin solid(e.g. paintbrush)
1. Start with your sacrifice dart and seperate the head from the foam. Make sure that the head is completely chopped off, you only want to cut to the stem of the head.
Not that this procedure is for sacrificing a streamline dart, so if you already have your streamline/ Elite dart head skip to step 4.
2. At this point the you most likely won't be able to get the head seperated because the dart's glue is quite strong. So get a thin solid like a paintbrush and try to push the head out shown in the picture below.
3. If pushing the head won't help, try twisting the head out of the foam along with pushing it out. Do be careful to gradually increase the pressure when you're twisting/ pushing the head out, or you could rip the foam.
4. So after about a minute of twisting and pushing, now you should have your streamline head!
Cut off the stem about two thirds of the total length like in the pics below. The head will mimic the weight of an Elite dart head.
Just for reference, here's a head I already prepared alongside the original head.
5. So now push your modified head back into the foam dart body and secure it with some adhesive like hot glue or other alternatives. I used sticky tape 'cause I'm lazy. :P
Do also cover the hole with some glue or whatever to improve accuracy too.
So there you have it, a converted streamline dart! As for usefulness, it has noticeablely more accuracy, BUT however, this does not work with clip (mag)-system blasters as it will jam the blaster very easily.
UPDATE: Here is is some better suggestions from P13c30fch33s3 and Rolley:
P13c30fch33s3: Gluing the tip back on instead of using tape will improve clip (mag) system compatability, and filling the tip with hot glue will drastically improve accuracy (at the expense of softness and potentially range for weak blasters). For a couple of darts doing these mods is fine, but when you get to hundreds of darts it can be really annoying, tedious and time consuming.
If I'm going to spend a long time working on darts, I'd much rather just get/make stefans and not mod stock darts.
Rolley: For mildly modded blasters I used to cut half of the head of streamlines off (like, half of the squishable tip) and form a dome of hot glue for the tip. It's more accurate but the trade off is that it is a hard tip that can hit harder.