Regarding the short barrel and powder burn. Before I cut the QLA from my X7 it had a 24" barrel, including the breech plug and QLA.

Here are 5-shot average chronograph readings from that barrel with T7 FFFG

250 grain Deep Curls

85 grains - 1897 fps
95 grains - 1912 fps
105 grains - 1939 fps
115 grains - 1976 fps

270 grain Deep Curls

85 grains - 1807 fps
95 grains - 1873 fps
105 grains - 1941 fps
115 grains - 1976 fps

300 grain XTPs

85 grains - 1749 fps
95 grains - 1817 fps
105 grains - 1875 fps
115 grains - 1915 fps

As you can see, velocity continues to increase with all bullet weights. I have to conclude that all of the powder is burned, even in that short barrel.