The 4mm bead is the one I used for a front sight. I have an old Crescent double with an ivory bead that's the same size. Three other double all have metal beads smaller than the NECG 4mm bead.

It's too large to use in combination with the 5mm bead in front.
Personally I would not want a front sigt any larger than the 4mm one (5/32").

BTY-Staining only took a minute or two, wiped it with alcohol let it dry then swabbed the iodine on and let that dry.

Ask not for whom the dog barks, it barks for thee...
NRA Life Member