There is a little exploit you can take advantage of early in the game, as soon as you unlock Trevor. With him, you can unlock all the weapons and gun upgrades for free. It is, of course, cheating and an exploit that most likely will get fixed in future updates, so make sure you take advantage of it as soon as possible. So here is the only GTA 5 guns cheat that you can use without disabling achievements/trophies:

The steps are really easy. In the video below, we are using Trevor during the Nervous Ron mission, but actually as long as you have the money, you can do it during any mission. So here is what to do:

1. You need to be IN A MISSION with Trevor (who has a ton of money once you unlock him).
2. Go to a Gun Shop and enter the store.
3. Buy every gun and upgrade that you want. Also buy grenades.
4. Go outside and use a grenade to blow yourself up. You will fail the mission.
5. When you restart the mission, you will have all the money and all the weapons you have previously unlocked will be available for free.

And this simple tutorial guides you through the steps of getting all the guns in the game for free. Some will say that this will ruin the game experience, so make sure you think twice before using this exploit!