IMO the problem with the mags is the velocity has to be high at IMPACT. A 300g xtp bullet launched at 2100 fps(very fast for BP) is only going ~1350 fps at 200yds. Only 1675 fps at 100yds. You have to worry about the entire range of velocity you will be working with, not just the first 10-50 yds.

Please see this chart for various XTP's (regular and Mag) and various velocities. You can see that the performance design of the 44cal 300g XTP is very wide from about 800 fps to 1900 fps, which is ML speed over 0 to 200 yards range. The 45 cal 300g XTP/Mag has a performance range 1100 to 2100, which to me is excellent for inline ML, no so good for sidelocks and FLs. Based on the velocities that Hoss published above the Mag would expand for the 300g. The do tend to puch thru because of the thicker jacket, but the also do not loose their jacket and fragment when shot hard and close.
Chap