There has been many a debate of this question.
The "Reload" action in an of itself means that you could have the 250/500 round broken into sections the size of the weapon "magazine".
I see, depending on the weapon, why 250 round bin is BS when its a 5 shot weapon like a sporting rifle & now you never burn an action reloading.
But, I also see the other side where Switching magazines = Versatility and can be argued as too powerful too.
I prefer the magazine side because it doesn't do anything normal shooter can't do, where as the 250 rounds w/o reloading is just not possible in your typical 5-10 shot Sporting/Sniper/Shotgun weapon.
One thing that I do think needs to be addressed is "ammo size".
I think things like Grenades should be a Single = 10 Shots of "Other" ammo that is normally purchased in 10 blocks.
And add to that that Assault Cannon ammo should also be at a 1/10 ratio.
I can see an argument for Shotguns & Heavy MG's too, but at a certain point I feel it would be getting too complex if you tried to come up w/ a ratio rating for every weapon type.
I think at most it should be something like Firearms, Heavy Weapons/2, Grenades/Assault Cannons/10 for rounds/bin.
So you'd have 250 Pistol ammo, 125 Machine Gun ammo, or 25 Grenades as all being able to fit into a Standard Mount of 250 Ammo.