Seems like a survival/zombie apocalypse/SHTF scenario.
It may seem like that, today, because we're all modern folks with planes, trains, and automobiles, and worse, cell phones...
but the idea of one ammo supply for your long gun and handgun dates waaay back.
Back to when travelling meant on foot, or horseback, carriage or wagon, and it took days or weeks to get between places where you could resupply, and everything you needed (other than food you could forage) had to be carried on your back, or your horse/mule's back.
In the flintlock days, long gun and hand gun ammo was the same, other than the size ball fired. Same "primers" same powder, and same lead. The only "specialty" tool needed for each was the bullet mold (if they were different sizes). Move up to the percussion era, and it is still all the same, if your long gun and handgun used the same size caps.
SO it was, for centuries, until we get to cartridges, and the different size cases used by the different rifle and handgun mechanisms. Now, one needed two different and non-interchangeable ammo supplies. The idea of only needing to pack one ammo supply for both guns had a long history, and made good sense. So, when rifles and pistols using the same ammo became available, it made good sense to a lot of people, people who spent days, weeks, and sometimes months living in the wilderness, with all their supplies carried by pack animals, which placed limits on the size and weight of what was carried.