Why are you drawing the point on someone carrying 300 rounds of .308 when you only advocate carrying 80 to 100 rounds for yourself?
If some dude is humping 300 rounds of .308 ammo, you'd best watch out because he's operating from a base, he's probably traveling lighter than you are because most of his long term gear is at the base instead of on his back like your's is, he's probably better fed and better rested, he's probably been sick a lot less, and he probably has buddies with him with a similar carry load and similar good health, rest, and nutrition. At least some, or most, of them will have military and/or hunting experience.
All in all, they could be carrying 8mm Egyptian Hakim's or old 7mm FN49's and you'd still be at an extreme disadvantage moving into their turf.