You are installing the bead on a rib. There will be a bit of space under the rib, between the barrels. You might get away with only a plug tap.
Do you have a #31 drill?
Generally speaking, cheap taps are a bad idea, and not a bargain.
You may get a 6-48 tap at a machine shop, but not the others. They will be special order at big dollars. Guns have a long history of using non standard taps and screws. It’s partly because the walls are so thin that they need an ultra fine thread pitch to get a decent grip. Brownell’s gets the weird taps in bulk which as always far cheaper than a special order. Also you can get tap,drill and bead all at the same time. For the price you probably want to get a couple of beads. Murphy’ law says if you only have one, you will drop it never to see it again. Ask me how I know.
I get the murphys law lol. I let my 3 year old shop partner watch over the original bead. Apparently she sub contracted the job to "barbi" who in turn smeared sparkly strawberry lip gloss on to it which in turn led to my mastiff eating it. Or at least thats the story im told. Either way i asked her where it went and shes telling me its in the dog