Having extra boxed ammo available to reload is pretty straight forward. Where you keep it, so that you can get to it if something bad actually happens is another story though. Even the LMG's only had belts of 200 rounds per can and that goes down the pipe pretty fast if the shooter has a heavy trigger finger... Have you tried lugging that stuff around in a shootout ?
I don't see the need for all the magazines. After trading fire for 2 or 3 mags worth, the bad guys better be dead, have given up and run to the neighboring county or they are going to be kicking your butt... The thought that one is going to have the luxury of holing up somewhere while pouring out all that firepower makes me think that said person has never been on the receiving end of even a fraction of the same firepower. Even 40 rounds worth of 308 is going to take apart anything but solid reinforced concrete. And if you can't carry it and move, you will be dead.
I have been on the receiving end of a very modest amount of firepower several times during my service and our vehicles were relatively land mine resistant and armored against all the small stuff but a few specialized AP bullets. The only thing that counts when you face a fearless enemy is accurately placed rounds. The misses don't count for squat. The whole "suppressive fire" thing makes me laugh. That is precisely the tactic our enemies use on us western wussy boys who have no skin in the game. We are the ones who are scrambling when some bullets come flying in our general direction....