But the thing about countersunk screws is that if for example you compare the heads of six 5.0mm screws from six different manufacturers you'll probably get five or six different head diameters...
That is undoubtedly true but as with any manufacturing process there has to be an ellement of 'tolerance' and with a 5mm screw that would mean that the head diameter could be anything between 8.6 and 9.8mm (Mach. Handbook 26th Ed. p1456) and still conform to the 'standard'. It doesn't mean that different manufacturers use different dimensions simply to be different and screws from the same source could also be different - even from the same manufacturing run.
As a site we currently use around 6 to 8k screws a week, at least at present (that's 30 to 40 boxes, but is well down from our peak usage). I find that within a given manufacturer there generally isn't much variation in size for a specific size of screw. So if you are using 4 or 5 boxes of 5.0 x 100/70mm screws a day from, say, Timco, they will be pretty much consistent box to box. Switch to Spax or Reisser or ForgeFast or ScrewTite and I can pretty much guarantee that the same (nominal) size screws will have different heads. And I specifically chose 5.0 x 100 because whilst most manufacturers use PZD#2 connectors, there are a few who use PZD#3 in this size (which generally requires a bigger head). We've noticed these variations particularly in the last few months as our supplier has struggled to deliver the quantities of fixings we need and so have at times sourced from wherever they can get them.
