Heads up! I'm going to turn you soft, white, good for nothings into real Guerrillas, if it kills me (again) to do so!
When we were riding in western Missouri-Eastern Kansas burning down Jayhawker Dens and putting bullets in Bluebellies waving that god forsaken Stars and Stripes, we came into a lot of different weapons that we had to use, and thus become familiar with. In the next Civil War, you'll run into the same thing. Therefore, you'll need to cross train and learn to use these weapons effectively against the enemy.
For example, talking out a ZOG sniper will leave you in possession of a Remington 700bolt action chambered in .308. Raiding a police station or an armory will garner you a whole mess of M16s in different "A" configurations.
Here we have our Friend again, (I don't think he's anti-racist) who knows his guns. The AR is a very versatile weapon, highly customisable, and ammo is unlimited. I would recommend as a minimum a flashlight and a holosight on the weapon. As for as the upper, I would personally get a 1-9 twist or (UPDATE: FASTER!) , as this will allow you to shoot standard military ammo (55grain) and heavier with good accuracy.
DPMS is a good choice for the money.
Here we have an invader that has had military experience in the ZOG Imperial Army. The reason I post this is twofold: One, to show you the correct way to insert a magazine into an AR (for those used to the AK, pay attention! Every nanosecond counts) and to show you that not only YOU have weapons and know how to use them. You must be BETTER.
Here we have a demonstration of clearing a malfunction.
Good luck, future Gorillas! Yeeha!