Admittedly, I never saw a huge difference doing this, and mostly participated in the system to keep myself under the 200 weapon inventory cap. (gamingenthusiast.net)
Get it from any Ammu - Nation location in GTA Online (in Story Mode, the Sweeper is automatically deposited into each character's weapon inventory with two full clips and additional ammo can be purchased at Ammu - Nation). (thatvideogameblog.com)
These weapon settings can even be changed mid-battle as the entire weapon inventory seems to be located somewhere in Hammerspace. (the-games-blog.com)