The sparks and flames ejected from the muzzle are the expanding gasses & red-hot chunks of the solid waste, not unburnt powder. The black specks on snow or a sheet are not unburnt powder, but solid waste. The figures I've seen for the last 40 years, are 57% solid waste. If you still have trouble with this, do the test, fire a number of loads over a sheet, gather up the 'unburnt powder', and use that for the next load and report on it's velocity.
Dan's note is not the first time I've heard of Norman Brockway pulling Ned's leg with the unburnt power deal.
Note that at some point, increased powder charges do not increase the speed at the same rate as lighter charges give. That means that at some level, the increase per grain of powder diminishes. Some people use that as the 'stop' point, caling that a maximum load. I choose to use the charge that shoots the best, gives the best accuracy with that ball/patch/ lube combination.
I have never found a gun's accuracy load to be less than what many call a heavy hunting load - no matter what the rate of twist. 38 years ago, my TC .50, with it's button .004" deep rifling with it's fast 48" twist, preferred 110gr. 2F GOEX or GOX or C&H or Meteor or ICI or Dupont or whatever it was I used back then. I used all of them, actually and all of them shot very well with that load in that gun. .022" denim patch and .490" ball - yes - very tight. I used 2f because it was more accurate than an equivalent (same vel) charge of 3f and the 2f also fouled less - probably due to having lower pressure which produces less demand on ball and patch combo - just a guess - my opinion.
Today, I use a larger ball - only .005" smaller than the bore, and the same thickness or thicker patches & I use the powder charge that shoots the best in that gun, not an arbitrarily chosen charge to save money, or it's what someone else uses, or it's probably good enough - I use that rifle's 'best' load & I use it at all ranges. I do reduce the plinking charge in one gun only, due to it's impressive recoil and wear on the person - the .69's 165gr. accuracy charge is a bit much for plinking. The lighter load doesn't shoot as accurately, but I am willing to put up with it, the little amount of plinking I do with the big bore, any more. According to the recoil formula, it is producing some 69 ft. pounds of recoil. A modern .458 Mag puts out something like 58. albeit with a higher recoil speed, which also tells on the body.
Here's the little 14 bore, with an accuracy load.