Twist drills do not make round holes. And, they often push metal rather than cutting it, if not maintained sharp. Have you never deburred a hole? Where did the burrs come from? Also, twist drills make holes bigger than their shank size. Round holes are made with reamers or are cast, possibly stamped in a die, as such.
The jet orifice is not simply a hole size. If you use magnification, you will see that the jet is ramped both into and out of the restriction orifice, and the orifice wall is rounded, not flat. The reason is that these forms help to keep the fluid flow in a laminar, non-turbulent manner over a wide range of fluid speeds. Stepped orifice walls will introduce turbulence, often in a modal fashion related to fluid speed or the pressure difference between the orifice inlet and outlet.
You've probably seen this effect with a garden hose nozzle, where at low pressure the outgoing stream is solid, but when the pressure is increased, an air pocket begins to appear at it's output. When the pressure is increased more, the solid stream begins to flail about wildly.
Drilled jets often flow LESS at holes sizes larger than smaller sized properly formed orifices, when subjected to high flow rates.
Precision, well formed, holes are seldom achieved with $3 tooling, unless you are not measuring, or not looking very closely.
It is possible to use jet reamers and jet gauges to get much better precision when modifying jet orifices. Still, these may not flow as well as properly formed jets. But, there may be no alternative if a suitable source can't be obtained.
I wrote the above before there was prior response.
Remember, you did ask. And, I am not simply trying to get on your (or anybody's) case, or, deliberately annoy you. FWIW
Cheers,
