Besides the suggestions what which weapons to buy and not to buy, here are some other weapon tips that are good to know.
If you refill ammo for your weapons in Ammu-Nation, there will be a very high chance that you accidentally purchase a gun you don’t need.
I would recommend the following methods to purchase ammo, which are safe and more effective:
Start a Contact Mission, and select “Purchase All Ammo” in the Mission set-up screen. This allows you to buy ammo for all your weapons at regular prices. Then, you can leave the Mission without starting it.
Open up the Interaction Menu, select “Inventory” and then “Ammo”, then browse through the weapons you own to purchase rounds for them. Notice that the rounds’ prices will be slightly higher than normal.
If you need to buy special ammo for any Mk II weapon you own, you have to visit a Weapon Workshop. Such process is also error-prone, but there is no alternative way to buy special ammo, so please be careful when browsing weapons in a Weapon Workshop.
If you own multiple weapons in the same grid of the Weapon Wheel, you can choose the default weapon for the grid by selecting that weapon in the Story Mode’s Weapon Wheel. For more details, please refer to this article on Rockstar Support website.
According to my own experience, this method has the following limitations:
Because there are no Mk II weapons in Story Mode, you cannot choose a Mk II weapon as a default one.
You must have unlocked and purchased the default weapons you want to select in Story Mode. If this does not apply to you, you may download and load a save in which all Story Mode Missions have been completed, or if you like it, do those Missions to unlock the weapons.
Occasionally, the default weapon selections are not applied after you leave a Job in GTA Online.