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Generally, the powder that produces the highest velocity in a long barrel will also produce the highest speed in a shorter barrel... the trade-off for keeping that velocity is muzzle blast.
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This is not true. In fact, for someone with 9,000 posts to say this is a little strange. If this was the case, then we should be using pistol powders in our rifles for every load. There comes a point when fast powders are ideal for short barrels because they will be mostly spent by the time the bullet leaves the barrel but not too good for longer barrels. Sometimes, a faster powder works well from a longer barrel as well, but often a slower powder is required to extract the very most velocity out of a round. Hence, the purpose of ultra slow powders for magnum cartridges.
6.5 grendel is a good example of this principle. For the heavier bullets TAC gets some higher velocities from longer barrels, but when you shorten the barrel you need a faster powder to get the best velocity from that barrel length. If you were to use the faster powder in the longer barrel you would reach your pressure limits before you got to use the whole case full of powder and you would end up with lower velocities or even worse a blown up gun if you fill the case.