There is a point where conventional fire-arms would break down and be incapable of use. As well, mass manufacturing would most likely restrict the availability of making ammo for assault weapons. There may be means in the world to certainly produce more, but I'd hazard that they're incredibly expensive and hardly worth using in general conventional warfare. You might use a repaired (hardly refurbished) AR16 when in a pinch, but given a lot of major automatic weapons are made for the purpose of expelling as many bullets as possible, using one for anything more than a few magazines or drums will eat into whatever resources you use for trade/bartering.

I've certainly used this excuse to produce a world with no guns. Though this often comes with raised exceptions where single-shot weapons may be the most practical fire-arm useful in that universe: they tend to eat through ammo slower, and their argument that they're a more accurate weapon means someone who can sight a good shot can more effectively use their weapon against many foes. In comparison to semi-automatic or automatic weapons where bullets may not travel reliability and their rate of fire makes them expensive to maintain.

And given the setting and the further you move away from the point that caused the apocalypse you run into matters where these become valid concerns in addressing the "material" plot-holes: more time means less goods and less parts to maintain existing weapons. The supply in finite and over time sustained use will eat through the existing supply, effecting the cost of the materials needed to effectively operate a fire-arm and making them more expensive. It's economics 101 really. Economics bred with survival.

Therefore in my mind it comes off as being both complimentary to the setting, and economically simpler on the character to use weapons that don't waste years-old ammunition. This usually transforms back to a world of crossbows and bows to supplement fire-arm usage. Or even makeshift swords. You might have communities that make makeshift fire-arms, but these would be comparable to old muskets and rudimentary, easy-to-use and build rifles. So now the actions are steered less from being Rambo to being Mel Gibson in The Patriot.

If the RP takes place immediately after I can see firearms being common as well as ammo. The stock of ammunition and parts needed to maintain a gun are all still at the level they were before with little noticeable tampering. But if the world is going to be all-out war all the time for resources I can see a lot of stockpiles being tore through very fast.

And let's take a minute to math on the numbers, for perspective. The AR-15 fire 800 rounds a minute. Because the AR15 is fairly popular as a privately-owned assault weapons, we'll assume it's the "average" of the roughly 1.5 million assault rifles out there ([url=http://www.slate.com/blogs/crime/2012/12/20/assault_rifle_stats_how_many_assault_rifles_are_there_in_america.html]Drawn from here[/url] which [url=http://fas.org/sgp/crs/misc/RL32842.pdf]draws from here[/url] [Even if the given statistic is 20 years old, but it's a rough start]). If all of there used at the same moment in one minute over 1,200,000,000 rounds would be fired at once.

I can't find numbers for the amount of rounds produced and sold. But considering that 2014 is the year we're just crawling out of an ammo shortage, I can't imagine there's enough pre-made factory rounds to be used for very long in a panicked situation.

So I see "no guns" as being less a rule against content (or an extreme form of balancing) but more a way of enforcing a rather absolute idea that there'd be such a level of extreme shortage in the world there's no point.