If you believe this is the end of the American Republic, then I urge you to buy a nice scoped rifle, 500 rounds of ammunition and become proficient in its use. I'd recommend a large caliber rifle in a commonly available round such as 7.62mm NATO/Winchester 308. Common rounds can be bought for reasonable cost in bulk.
No need to go to buy a new rifle. A pawn shop would have used hunting rifles available thus saving you some money. A common and very reliable rifle is the Remington 700 series.
A handgun is good too. While a 9mm or .45 is great for self-defense, as the Mafia is known for, a .22 behind the ear is better for assassinations. It's quieter and the bullet only penetrates one side of the skull then rattles around in there finishing the job. A .22 revolver is very reliable.
I have not bought a gun, yet. I live in Pennsylvania, and have no criminal record, so it would be very easy for me to buy a gun. I am naturally frugal, but do not need to be. I could buy a new gun, and even a more expensive gun, without it making much of a dent in my budget.
It would make traveling over borders more difficult. Foreign countries will often ask Americans if they own a gun, and if they do there is heightened scrutiny.
I am not saying I will buy a gun, but I did write down your advice. With handguns I have always been partial to the 9mm, like maybe a Glock 26(as an engineer, I like Glocks). 9mm is a widely used standard, which tends to mean that it works best. The .45 was literally designed to kill horses(a much bigger creature than a human). I cannot imagine needing to defend myself from a horse. I have no problems with horses, and my daughter would never forgive me if I killed a horse.
Revolvers are more reliable, but have a slower action. You can get off 10 rounds from a 9mm stacked semi in the time it would take an average shooter to shoot 2 rounds from a revolver. I have seen some revolver shooters who can shoot amazingly quickly, but they are rarities.
The NYPD did a study of police officers shooting people at two yards or less. These are reasonably trained shooters, who can reasonably expect to have to shoot someone, shooting at a distance that a blind shooter could not miss at on a shooting range. They completely miss the target more than two thirds of the time. And we can assume that some of the times they hit the target, it was a minor wound. In the emotional, scary circumstances of shooting someone, even hardened and trained police officers miss a lot.
So I would want something that would shoot as many shots as it would take to hit the target. Even odds of one third per round, ten rounds have over a 98% chance of hitting a target. A Glock has a jamming rate of one in 5,000, where as a revolver is 20 times better, but one in 5,000 is plenty good enough. The odds of one of those 10 rounds jamming is 0.2%.
I am not an expert on any of this, so if anyone thinks I got something wrong, feel free to tell me. And again, I have no immediate plans to buy a gun. I am clearly thinking about it.
Conspiracy Theories
