@jmorris I guess maybe less likely to break from chip packing? They readily evacuate a long string chip out the top. The cheapest ones are still cheap of course. I use tap magic most of the time for tapping, too.
I'm trying to remember the smallest I've done. I own an M2 spiral flute tap and it's not broken, but I do so little that's smaller than M3. And the M2 spiral flute tap I own isn't the highest quality.
But yeah, spiral tip should be stronger than spiral flute, and eject the chip ahead of it as long as you aren't tapping a blind hole. (I do plenty of blind taps which is part of why spiral flute taps are my go-to one-and-done.)
@jmorris Oh, #4-40 is only .155mm smaller diameter than M3, so maybe my good experience with M3 carries over...
I do almost always use a tap follower or some other effective guide. I think the spiral flute taps are not at all forgiving of misalignment. But I'm usually doing it on mill or lathe where I have a good normal reference.
I did also make 3D printed jigs for tapping openbuilds v-slot with M5, where I can't typically use either mill or lathe to get that normal reference.
@jmorris Glad you got it working!
Tap guides are a reasonable form of vertical reference; mill not required. That's why I mentioned what I do for tapping v-slot. They keep the taps straight enough that I've (touch wood) had 100% success power tapping with a drill and a spiral flute tap.
Also, not trying to talk you out of form taps. I want to go in the opposite direction and start using form taps where appropriate.