compressed air quick disconnects are fucked up like a football bat

 and so are small NPT* pipe diameters.

I want to write more about this, but I am mentally and emotionally and physically exhausted.

So maybe I'll do that later, so I guess this is now a placeholder so I don't forget.

Ahem. >_>

...

 Okay, it's a little later. I'm eating lunch. I've taken some deep breaths.

 Given: quick-disconnect fittings are in common use for compressed air hoses used for air-driven tools, blow-off nozzles, spray guns, and so forth.

 Item: in the USA†, there are not less than two commonly-used interface patterns, each of which is available in two sizes; and there are not less than two minor players (which may have better flow performance, but which are not as widely available) and those also come in two sizes!

 Item: pipe and pipe fittings commonly used with these QD fittings are sized by their nominal ID, but their as-manufactured ID can vary wildly and may be larger or smaller than specified.  Note that smaller IDs represent flow restrictions.

 Item: common air-hose sizes found around the shop and attached to man-portable compressors come in 1/4", 3/8" and 1/2" ID sizes.

 But the fittings!

 The 1/4" ID hoses usually come with 3/8" trade size fittings, so that's fine.  But the 3/8" ID hoses also come with 1/4" fittings.  You would think this would be a significant flow restriction... but it turns out that most of the 1/4" fittings have a nearly 3/8" ID, and most can be drilled or bored out to nearly 3/8".

"Whoa!" I hear the safety-smart among you cry, "won't reducing the wall thickness reduce the pressure capability of the fitting, potentially making it unsafe?"  Well, yes and no.  Yes, a thinner wall will have a lower pressure rating.  No, you won't make it unsafe - unless you get carried away - because these fittings have a minimum safe working pressure of 175 to 300 PSI, which is approximately 2X the burst pressure at which it will actually fail. I don't run my system higher than 100PSI, in fact the regulator for the whole system will be set to 90 PSI with another downstream regulator for the tire hose set to 50 PSI. The high (tank) side is set by the compressor cut-off switch at 125 PSI.  The compressor tank's safety valve pops around 135 PSI. So I am sacrificing theoretical pressure capability I don't need for reduced flow restriction which I do need. Use common sense and don't quibble.

 The 3/8" fittings have an ID ranging from 5/16" to a full 3/8", depending on the quality of the fitting.  Boring these out when they are undersized removes very little material.

 1/2" hoses come with either 3/8" or 1/2" fittings.  1/2" fittings mostly seem to have ID's close to 1/2".  (trade size matches nominal size)

 Larger pipe sizes seem to be closer to their nominal size.

 Item: the two connector shapes are variously referred to as:

    I/M - Industrial / Milton™ pattern

    A/T - Automotive / TruFlate™ pattern

 The two do not intermate.

 When it comes to flow restrictions, the valve inside the female QD (which automatically shuts off the air when a tool is disconnected) dominates.  The bigger we make those parts, the less the turbulence through that valve will slow down the flow.  I chose to standardize on the larger - 3/8" size for flow reasons, and on the TruFlate style because I suspect the TruFlate locking ridge is less subject to damage - but that's a quibble, and loads of people have had years of good service from both styles.

 Avoid generic or cheap zinc connectors.  The premium connectors are the name brands, including Milton, TruFlate, AmFlo, and the top shelf would be Prevost™ hoses and connectors.

 Allegedly, AmFlo makes universal connectors capable of mating to either of the other types, but I've not seen or touched them, so I have no opinion on them.

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*for my non-Amurricun readers, NPT= National Pipe Thread, the main pipe thread standard
used in Amurricuh for gas, water, and compressed air, among other things.  It is similar to
British Pipe thread, but hsd slightly coarser TPI and NPT is tapered whereas BPT is not.
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†outside the USA, you may have to choose between "German" and "British" patterns