I understand that but when you have these massive timeline divergences , there is just not a reason to use real firearms.
Why wouldn't there be?
If society/civilization collapsed today, right now, or a huge war crippled the world right now, we would pretty much be stuck with the weapons we have. We might see remnants of government military forces winding up in possession of the most advanced/current/modern weapons, but the ordinary citizens would retain what they already have or what they could easily obtain.
Basically a Fallout 3 scenario in the USA would involve weapons such as the Remington 700, Marlin 336, Ruger Mini-14, AR-15, AK-47 (semi auto versions), AK-74 (semi auto versions), SKS, M1 Carbine, M1 Garand, Mossberg 500, Remington 870, Glock series of pistols, various Beretta pistols (most popular being the 92FS), etc. Basically weapons that would be useful for property/personal defense against multiple attackers and weapons that might easily be in the hands of local/county/state police forces.