I'm very much interested in this topic too, thanks for posting it.
The term "practical rifle" in my mind is the use of a rifle for defensive purposes. Once you mentioned .300 Win Mag I'm not sure what kind of practical rifle craft you have in mind, so what follows may not pertain.
All competent training is good, but in my opinion each of us have to adapt what we learn to our specific circumstances. The chances of me having to use a firearm to defend myself is pretty unlikely, and the chances of using a rifle to do so are mighty remote. If a deadly force self-defense scenario unfolds for me, it will most likely involve the pistol I'm carrying.
While learning to "run a carbine" rapid-fire with multiple magazine changes at pistol engagement distances is a good skill to have (it seems the vast majority of AR training is based on this), I think of "practical rifle" more in terms of using the AR at 50+ yard distances from cover and a variety of improvised positions (over the hood of a car, from behind a tree, sitting, kneeling, prone, etc.). I also believe that for the civilian, practical rifle craft should leverage the main advantage of using a rifle - the ability to make quick and accurate hits at long(er) distances. In my case, I focus my skills to do so in the 50 to 200 yard range.
This line of thought then drills down to the equipment level. There are some very obvious trends in AR accessorizing that nearly everyone seems to follow, but defining the role of your "practical rifle" can help avoid a lot of unnecessary equipment expenditure and experimentation, and help identify what skills need to be built/mainatined.
Great topic but I truly believe it needs to start with an honest assessment of what the intended role of the practical rifle is for you. My apologies if this is outside your intended discussion.