Re: Guns + ammo, lack of electricity
Ammunition is unlikely to be a major constraint.
Those people without guns will be in a dire situation... but they won't need or consume ammunition.
The average sport shooter will have hundreds to (more likely) thousands of rounds, as they are far less expensive when bought in lots of 1,000 or more for each caliber, while the average serious hunter may well have hundreds of rounds.
Preppers will have thousands to tens of thousands of rounds, stockpiled by the case (typically 1,000 to 2280 rounds each).
This is easily enough to clear your local sporting goods store or gun club of ex-employees, in order to access their storeroom. Larger quantities are likely available at major police stations and military bases, unless they have been used to eradicate zombies, which cuts down on both ammunition and zombies.
Those with good research skills can hunt down distributors' warehouses.
I do agree that the collapse of production and distribution (something electricity loss is a cause/proxy for) would kill a lot of people until the population density dropped to the level supportable by hunting/subsistence farming.\
The study, however, seems to assume that people have no tactical sense, nor adequate shooting skills. Those people will die first, the rest will be a lot more effective.
Note from the above... ammunition is heavy. Most sport shooters and preppers would require a vehicle to relocate their stockpile... even light ammuntion, such as 5.56 Nato, would max your load out with no more than a couple of thousand rounds, and you wouldn't like the weight. More realistically, you can only carry several hundred rounds. Note to zombie apocalypse participants... avoid shotguns, the ammo is way heavy and too bulky. The lightest effective rounds are the best, where effective depends on skill as well as raw power.