See your point about trying to stay within the AR platform. I do belong to the school of thought that says carry a few less but more effective and deadly rounds. It is my understanding that the 5.56 rd now is not as effective because the bullet is more stabilised and does not "tumble" on impact and therefore goes right through the target without making a big wound channel. I discovered what you are speaking of about the short powder column and accuracy sometime ago with my 284 win. The rifle(a mod 77 Ruger) was chambered for 7x57. Very pleasant cartridge to shoot but I couldn't find any loads it would shoot accurately. I took it to Ed Shilen and he rechambered it for 284 win with a deep throat. (sounds kind of obscene) I started working up loads with the 140 gr Nosler. The more dupont 4350 I used the more accurate it shot until I settled on 58 grains and that produced the 3100 fps. It will shoot min of angle 3 shot groups off a bench with no signs of pressure. The 284 win brass is very durable and a joy to reload. hardly any resizing necessary. Unfortunately I no longer hunt big game so a great little mountain rifle sits. also has a Canjar trigger and 2x7 Leupold.