I've become aware that the reason 8,10 and 12 inch barrels are so popular in 300 BLK is at least partly due to the fact that
300BLK loads usually use pistol powder so that they can burn clean with the reduced barrel length.  I don't see barrel
length standards when velocity specs are given for this round for just about any ammo sold.  So the question is, if I have
an AR upper with an 16" barrel is the off the shelf 300 BLK ammo sold by most manufacturers not optimal and geared toward
shorter barrels?  I have not been able to find much info on this.

Yes....and pistol length gas tubes have worked best for me in 300bo

MOST load data velocities I've seen are all based on 16" barrels. The one I've seen that wasn't stated 16.5" barrel suppressed. Since 16" is the BATFE minimum (without paperwork) that is usually the minimum.

If they are using 9" barrels and getting 2200 fps, I'd like to see what 16" gets.

It's because you need to post this in the reloading forum.  Ammo forum isn't the same thing as reloading forum.

This is indeed the question, especially with pistol powder.  Would it be less since expansion may be nearly complete by about 8-10" and the bullet is essentially
dragging down the last 6" of barrel ?  Could this be why custom loads seem to be very popular with this round?