The supposedly H&N copper coated lead BB's (they are a definite gold colour) I got from Protek average out at about 4.34mm - but because they are lead they do appear to have tiny flats around them - perhaps as a consequence of being supplied in bags of 1,000 and the various handling stages before they got to my doormat & the actual diameters vary between 4.32 & 4.36mm.