I might be able to explain. This is one man's take on it, and don't take this as fact, please. Just as the musings of a man with a game to look forward to in 11 hours time.

The Straightline is made using the same methods used to make regular Hammerheads. Gun drilling and all that. This is a quality manufacturing process. It's bound to result in a slick, minimal friction inner surface. With less friction, it takes less gas for a push of the same force, or the same amount of gas can push with more resultant force, thus the increased speed.

In addition, due to to rifling, not all of the barrel is touching the shot. The parts of the barrel which do touch the shot are the "lands" (raised areas) of the internal surface. Less contact equals less friction.

I also think that the fact that there are air channels around the shot (the "grooves" of the rifling" allows air to gather around the shot, giving it a cushion of air to float on which reduces friction.

Remember, all the above is just speculation. All my points may be right, then again, all may be wrong. I do not own the Straightline, or a Hammerhead. I suffer in silence with a stock barrel. The above is mere conjecture based on fact. Caveat lector.

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