I've done a bit more research on other sites and I tend to agree with the general conclusion that, unless we're talking about unusually short barrels, it is likely that you will never come-up with a load of fast burning powder – in a given caliber – which provides more velocity than a safe load of a slower powder in that same shortish barrel.
It does look, however, like you can reach a point where the velocity differences between the two are pretty insignificant; whereas the slower burning solution might yield a lot more muzzle flash and concussion – potentially making the faster burning load a more-preferable trade-off for shooters who care about such things – like me.
As an aside, the video suggests that you can come-up with a “less blasty” gun with higher muzzle energy, if you go so far as to transitioning from one caliber to another (say an 8” 7.62x39 verses 8” 308). Interesting, but as noted, It looks like unlikely that you can achieve this within one caliber, just by optimizing powder choices, in relation to barrel length.
IMO, none of the above takes-away from the conclusion that picking the right powder for the applicable barrel length is important. To pick an intentional-extreme example, I kind’a think I’d use a different powder in a 2½ 357 magnum pistol versus ammo loaded for an 1895 Marlin, in that caliber.
I also agree with all that QL is a useful predictive tool – but that everything has to be validated by testing. I know that Evanguy was planning to do some testing with comparative loads obtained by another poster – in a very valuable no. Mk. III. I’m sure that all will be interested in the results.
You brought-up the matter of sound meters. I bought one from the Source, a while back, and have found it to be a useful tool (LINK). The mods will forgive me for mentioning the product of a non-sponsor – as that item is now out of stock (as such, no plug provided).
Similar devices are listed on the web, starting at around $25 - for what seem to be decent items.
I’m an Android guy and it look me seconds to locate and install a free sound meter app from the Play Store (one of what appears to be a list of about 50 such apps, found there).
I have used the real sound meter (from the Source), but not used the app in field testing. For all I know, the apps may be more for testing noise levels from traffic in your neighborhood, etc. - and might not have the range to handle gunshots. And, I suppose it is possible that a smartphone could be damaged by capturing the sound from a nearby gun blast. Buying a cheap, dedicated sound meter might actually be a better choice. Otherwise, in order to try things out, maybe you could borrow a smartphone from another of the posters – I can suggest a few.
In Quicklo*d's lower right quadrant, is field entitled “Values when bullet base exits muzzle”, … “muzzle pressure”. This is the field that you want to minimize, to achieve a nice-shooting load, in a shortish barreled rifle – all other things being equal.
I wouldn’t be surprised if loads with a low number in this field could be accurate loads; since the exiting bullet wouldn't be as disrupted by the forces of a lot of high pressure gases, struggling to get past the bullet – just as it leave the bore. Just a guess there.
BTW lots of bench rest guys use guns with barrels less than 16" so this development work could be useful to others.
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