In my experience, 1/2" groups at 100 yards is within the capabilities of ANY cartridge when built with modern quality components and fed tuned handloads. I've not shot a gunsmith chambered barrel that wouldn't do 1/2" or less at 100. Most do 0.25" or better, and that includes hunting rifles. Feeding a "short" 284 in a LA from the box magazine is going to be fine. Throat the barrel long to gain as much case capacity as possible if you want to maximize your speed. My grandpa built a 300 Savage on a Mauser 98. That's a true short cartridge in a medium-long action and it feeds like butter. I load the rounds all the way at the back of the magazine box and have been able to chase the lands without any feeding problems.