I agree with you 100%. This is part of a system. It is a bag gun. It goes in my pack. On my shoulder is an ar. On my hip is a hand gun. In my wife's bag will be a .22 Marlin break down. I don't plan on 100 rounds, just 25.
Not to be negative but, If those are different caliber weapons which I assume they are and your carrying three days food and bagged or bottled water, energy bars for emergencies, cooking equipment, fire starting at least three ways, a cpl tarps at least maybe a small two person tent, a blanket or sleeping bag, some type of hydration/filtration system, medical gear, clothes maybe a poncho or coat, flashlights, compasses, a radio maybe a cpl two ways, ax, machete, survival knife, plus three calibers worth of ammo, I know most people think its a lot and with two people in the group you can just divide it between two packs, but you really cant you have to remember each pack has to be a complete survival kit unto its own because what happens if one person gets lost or you miss each other and have to meet up at the rally area, then your both left without the supplies you need to survive.
I may have forgotten some stuff but you get the idea, by using three different calibers your forcing yourself carry way too much ammo. Unless your longest hike is from the house to the car/truck in the driveway your going to either not have enough ammo or kill yourself with the weight. you might look into my solution a 9mm handgun and a 9mm tactical rifle. I also try to keep all the weapons in our group uniform i.e. everyone carry's 9mm sidearms and all smaller caliber rifles are .22lr. so between 6 of us we have a good mix of weapons for hunting/security but only use 4 different types of ammo 9mm, .22lr, .223 and 12 gauge. Anyway just a thought you should go hunting with your BOB a cpl of times to make sure you can deal with the weight or even just camping minus the guns but with the ammo packed...Good luck to you