Some weapons in Battlefield V have different rates of fire that you can toggle off an on. If your weapon does have multiple fire rates you can switch the fire mode by pressing the Y/Triangle button. This button also makes you switch between your primary and secondary weapon so you actually need to hold Y/Triangle to switch rate of fire.
If successful you’ll see an icon in the bottom right corner of the screen that switches between 1-4 bullets depending on the rate of fire you choose.
You can see whether your gun has different fire rates by looking for that icon next to your ammunition. It will look like a group of bullets in a row. For fully automatic firing all of the bullets will be colored white. For single shot there will only be one bullet colored white.
Just keep in mind what fire rate you have on your weapon as it’s pretty easy to die when you think you’re in automatic mode and you go to squeeze the trigger and a single bullet comes out.