sure but as long as one is with the sights it would be OK. Also you could use see through mags and have a line on one cartridge which would show what was in the chamber at that time.
One would have to hold under with the 55 grain and be on sight with the 68. For anything closer than 200 yards it probably would not make a difference.
Yeah, but if you fire a two round burst, you’re looking at firing two projectiles with significantly, even radically different points of impact versus your point of aim. That’s not a good thing. If you wanted to hit your target and nothing but, you’d have problems. I also can’t see that a soldier or Marine under stress is going to remember to check their mag to see which round they have in the chamber.