I would like to replace the front sight on my CZ550 with one of the ones offered by NECG (the White bead version).

Since the white bead offered by NECG has a thicker frontal diameter, what is the correct way to go about measuring my current front sight to replace it with the NECG version?

Thanks in advance!

It sounds like you want to know how to measure the diameter of both of the beads, divide by two to get the radius, then subtract one radius from the other to find the difference. That would work but the problem is that sights are not measured and sold in thousandths of inches or .01 millimeter increments. In the USA they are typically sold by the 1/32nd of an inch and in Europe by the 1mm or .5mm. They are measured by the total sight height as shown in the picture. The best method would probably be to measure your old sight and get one the same height. You can do the math if you like and see if you can get closer to going one size higher or lower but generally you have to make a rear sight adjustment of a smidgen or less.

front sight by Rod Henrickson, on Flickr

The CZ 550 Safari magnums come with 2 sizes that I know of, they are 1.5mm/1/16", which is on ALL standard rifles, and 3mm/1/8" which are on custom shop big bores in 450 Rigby, 404 Jeffrey, 500 Jeffrey and 505 Gibbs. The heights are identical, it's only the diameter of the bead that is larger.

Hope this helps.

Cheers.