I was asked, so here goes:
"Such tight tolerances..." What does that even mean? (nothing other than a poor attempt at trying to sound cool)
In their defense (Defiance), the 8-40 pitch is typically not listed in reference material like the Machinery Handbook because it's a very uncommon thread size/pitch used almost exclusively in the firearms industry. That said, taps for this pitch have been made for decades, and there are off-the-shelf thread mill tools and gauges.
The issue one discovers is that there is a significant fluctuation in the major OD and pitch diameters from the companies making this hardware. It is my guess that Defiance has discovered this as well, and this is why they push for people to use the hardware they supply.
An example:
I buy bags of Torx head screws from an optics company. I get them 5,000pcs at a time. If I thread a receiver to fit my 8-40 GO/NOGO plug gauge, those screws fit absurdly loose. They will rattle around like a marble in a can. So, I "comp" the tool in the machine and undersize things by a couple of .001s so they fit correctly. If I use a screw sourced from someone else, it's just as likely that the hole/threads are now undersized, and the fastener requires an unreasonable amount of force to install.
This is one of the little annoying/intrinsic "tribal knowledge" type things that my staff has to be cognizant of when doing receiver work, as it's very application specific. If you take the ten most popular companies making scope bases, I can promise you that the screws are NOT all the same. We have to be very, very careful when using taps for this reason, as there is no means of adjusting the pitch diameter with a tap. Over the years, I have acquired a large inventory of hardware to address this sort of thing when conducting repairs or installing stuff where I don't have complete control of everything.
If I am to offer anything constructive to this discussion, it's this:
Firearms are the wild west in terms of dimensioning. There is no regulatory body here in the US that governs how things are made or dimensioned. SAAMI, at best, is a polite suggestion. A 30-caliber bore dimension is a prime example. A "tight" bore 30 was born because it was discovered that a particular bullet made in England (the 155grain pill used in Radway Green machine gun ammo) performs best from a .297/.3075 land/groove diameter. The ammo is junk and has no business being shot from anything other than a belt-fed machine gun. However, the international rules for Palma state the host country is who supplies ammo for the team portion of the events. Radway is what you get when you go to England. (a few others use it too)
-How many well-intentioned target shooters buy tight-bore 30-cal barrels because the assumption is made that it's better just because the word "tight" is used in the product listing?
-A lot.
Hope this helped.
C.
