The title of this essay may be somewhat misleading, as it is not about flat-bottomed burls, but about tamping for flat-bottomed tobacco chambers. The pun on the Queen song was just too irresistible.
Tobacco chambers come in different shapes. Artisan carvers tend to make their own spade bits for drilling the chamber, so they're all a little different.
Tobacco chambers are generally cylindrical until near bottom, when they become a soft "V" shape. Some taper all the way to the heel. Some, especially prevalent in the Pot shape, have broader, flatter floors, and that sometimes makes it difficult to smoke all the tobacco in the chamber.
For a long time, I avoided pipes with flat-bottomed tobacco chambers, finding that "V"-shaped chambers provided me with more ease of smoking. Flat-bottomed chambers needed more finesse and care when approaching the end of the bowl. The tobacco tended to burn unevenly for me in flat-bottomed chambers — until I altered my tamping technique.
Tobacco tends to burn more evenly at the beginning of a bowl than during the last third or so, the primary reason being the location of the draft hole on the near side of the bowl. Obviously, pipes with an air hole centered at the bottom, such as Calabash pipes, are different, but for most briar pipes, the smoke hole is located at the side of the bowl, more prominently so with flat-bottomed chambers than with "V" chambers.
The result of this necessary design characteristic is that the smoke has further to travel from the far side of the chamber to the smoke hole than from the near side of the chamber. See the diagram below:
It isn't much of a difference at the beginning of a smoke, but the ratio suffers as the distance from the burning layer of tobacco to the smoke hole diminishes. Halfway down the bowl is approximately where the disparity becomes noticeable.
Because the smoke has further to travel from the far side of the bowl than the near side, the near side will burn a bit more quickly, leaving a disproportionate amount of tobacco unburned on the far side. It catches up, but unevenly. In a "V" chamber, the ratio remains more even, though there is still some difference in the distances from burning tobacco to smoke hole, as seen in this diagram:
I believe that some pipe smokers unconsciously compensate for this uneven burning characteristic by tamping more firmly on one side than the other, which can have an effect of making the tobacco burn a little faster or slower, according to just how tightly one presses. The guesswork can be removed, though, by altering your tamping technique to accommodate the angles involved.
Here's what I do, and maybe it will work for you, too. As the tobacco burns down to about the halfway mark, I start to tamp at an angle, pulling from the far side toward the near, so that the distance from the burning tobacco to the smoke hole is more even:
If you're careful not to compress the tobacco too much on one side or the other, you should be rewarded with a nice, even-burning smoke all the way to the heel. If your dottle tends to remain particularly wet, though, you may not want to try this, as it could damage the tobacco chamber if you apply enough flame to evaporate the built-up dampness. Either dry your tobacco more or save this technique for those tobaccos that smoke more on the dry side.
What happens when you leave the tobacco level near the bottom is that the near side tobacco burns down to the smoke hole, leaving tobacco on the far side. This generates an ashy taste and unburned tobacco on the far side. You might get some wispy smoke from it, but that's from air crossing over the top of the unburned tobacco rather than sifting through to the bottom.
I'm not entirely sure why this method works. Perhaps I'm scooping ash toward the smoke hole side and slowing the burn there so the far side can catch up. Perhaps I'm scooping tobacco so there's more to burn above the smoke hole, generating a better proportion of tobacco distance from the smoke hole. It's probably a combination of those. But either way, if you've had the same trouble as me, this tamping technique may be worth trying.