The game's entire setting is retarded. Losing knowledge on how to make gunpowder after 25 years of so called social collapse? Retards wrote this game.
To be fair, modern ammunition with smokeless gunpowder is reliant on a very complex supply chain that wouldn't be easy to maintain in the event of a near-total collapse of civilization.
Most post-apocalyptic settings tend to treat guns as rare artifacts of a lost age, when in fact the procurement and production of guns wouldn't be much of an issue.
Modern cars, appliances, buildings et al are full of metals and alloys that could be scavenged and used to make guns, - the issue would be the manufacture of bullets. Modern armies spend a lot of money and energy on the conservation of ammunition, since the ammo needs to be stored in a controlled environment and periodically rotated to ensure the propellant is evenly distributed. What this means it that soon after the post-apocalypse a lot of unused ammo languishing in storage would become useless.
As for the manufacture of new ammo, you would need brass to ensure a tight seal and minimize damage to the internals of a gun, but you can make things work with other metals - it has been done before. The biggest issue is going to the gunpowder. You can make black gunpowder easily enough if you have access to the raw materials, but when used it creates a lot of smoke and fouls the gun very quickly, and there's also the issue of load tolerances. If you load too little gunpowder there's a chance firing the bullet will not generate enough energy to cycle the gun mechanism properly, put in too much and you risk the gun blowing in your face.
This is an even bigger issue with fully automatic weapons that require very precise grain loads to keep working without jamming - so while making a full-auto SMG with redneck engineering is easy enough, actually producing ammo that lets you reliably fire it in full auto is not.
The first game's combat was trivialized by the introduction of guns: from a competent 1st person parkour + melee combination the game became a mediocre fps.
They could have circumvented the issue easily enough by handling it in a similar fashion as the Metro games - full-auto guns require rare and expensive per-collapse ammo, while contemporary guns are all semi-auto, loud, create a lot of smoke and are unreliable.