Some will tell you it's a 3-56 thread (even Winchester customer service told me that) because that's what Winchester used to use on their American made shotguns. Ya know, being made in the U.S. they'd use a U.S. standard thread size. But that ain't true. I recently lost the bead for my SXP and, thinking it was a 3-56 like I was told, bought that size replacement from Brownells. Alas, the 3-56 is too small.
Winchester's new foreign made guns use metric thread sizes. The SX3 and SXP I believe are M3-.5mm thread size.
I don't know about fiber optics, but i personally couldn't find a standard bead in a metric size on any of the major parts stores. So I will simply drill and tap it to a 6-48 thread, a common U.S. standard that you can find beads for anywhere.
