With most firearms a few good dry fires will tell you if it still works.

You will never find a store of firearms post apoc, but you will find clusters.

For example, Bob the Redneck, Joe the Cop, and Great Aunt Sally.

Bob the Redneck has three shotguns, four long guns, two hand guns, and an ex-wife Debby with the kids. When society falls and he goes to get little Bubba and Suzie and put a round in Debby (whether she is a zombie or not!) - he is likely to leave at a few guns in the gun safe at home - or he is going to pile them all in the truck. He might have 9 guns, but he can only carry a few at a time for use. In his gun safeor the back of his truck you might find a few guns in great condition. On his body in Debby's front yard (she has a gun too), you likely wouldn't find much worth keeping after a few months outside.

Joe the Cop is on the job when it goes down. Do you think he doesn't have his own guns at home? Zed overrun his position, he goes down. He is single, so his personal arms are in his home waiting for someone to take.

Great Aunt Sally has Great Uncle George's who gun collection (dozens of arms). Zed comes and she closes herself in the house and shoots zed with the keen eye an old woman with nothing to live for anyways. A few days later she runs out of lipertor and has a stroke leaving a home full of guns, but very little ammo left.

There will be cases of abandoned and unused weapons. There is no reason NOT to take them. Even if it is a random rifle (say, a 284win or 250sav), where you are likely to never see another round after the 12 or so left in the box with the gun - that is still 12 rounds - great for a man up on watch where the guns see a lot of weather but not much use.

If someone comes by for trade and has some of those ammos (scavanged without a gun to shoot them in) - then you have something of value (or want something they don't value).

 out of  found this valuable. Do you find this valuable?