Honestly, as someone who shoots alot, I wouldnt make room for extra bullets, at the expense of ditching other items.
I would offer this advice on carrying firearms; carry what you can COMFORTABLY. Me, I have a chest harness that carries 4 5.56 magazines, 3 pistol magazines, and a couple of small pouches for a flashlight, and notepad/small items. Loaded, this weighs maybe 4 lbs, give or take. IF there is an event that I would need to leave, and would consider my own personal safety, I would grab this & go. I keep them loaded, as I go shooting every weekend, just about. I also have a few additional rifle mags, that I could easily throw in a bag, no worries. But, I wouldnt carry anything more than that-you can get weighed down REAL quick carrying ammo-and, its for DEFENSE ONLY-you arent looking to ambush people, or anything like that. My loadout, when I have a hot mag in both a rifle in a pistol, is 40 rds of pistol, and 150 rds of rifle-that would see me through almost anything I can possible imagine.
MY BOB is scattered among several kits, often with redundancy. Between my normal hiking backpack, my work pack, my jeep, and my dayhiking pack, I have enough to cannibalize & make do at almost any time. None of them contain firearms though-as stated above, thats an entirely separate kit for me. My packs are designed for me to live, short term-be it moving to another, better location, or to survive a night or two without better equipment, or, when kayaking, to make camp wherever I end up in the event I separate from my boat.
Again, I am not against carrying a firearm with your BOB-I just think its a separate issue-I wouldnt dump gear to add ammo, nor would I carry a 75lb pack half filled with it. Firearms are defensive only in these cases-you arent in the military, so you arent expected to be in a prolonged firefight, if at all.