Many if not most pistol magazines have witness holes, making it easy to see how many rounds are left. I haven't seen such holes in AR mags yet, and being still a newbie I wonder if it would be a good or bad idea. I lost count when loading up my new plain aluminum mags; I had to count out 30 beforehand and then load up. I thought maybe a hole for every 5 rounds would help. Then again, holes could let in dust, grit, etc. Did I just answer my own question on why AR mags lack witness holes?