The Barnes is a purpose built bullet, designed specifically for the 300 blackout. It is designed to expand at lower velocities... down to 1300fps, and its longer tip and ogive enhance feeding in ar rifles.
I picked it because there is lots of data on it in a variety of barrel lengths. Hornady 110 vmax can be driven a bit faster, and has a comparable muzzle velocity. The highlighted sections correlate muzzle energy from a given length barrel, for both loads. While I can't comment on the exact reason the m193 was chosen for this particular chart, I would assume its because it offers the best average performance across the entire spectrum of barrel lengths, and because most readers will be familiar with the load. Mk 262, M855 or even XM777 will show the same relationship until you get to radically heavy or light projectiles.
Put another way, a 45gr loading will show less fps lost per inch at shorter lengths compared to a 77gr loading, and the energy will follow. While the barnes 110 loading is one of the lightest for the cartridge, this bullet weight will probably be the most popular weight for supersonic loads... just like 55gr 556.
You might head over to http://www.300blktalk.com/forum/ for more info, if you're interested.
Of course with the caveat that you will find more fanboys there than here.