This should come without much effort. Whenever you see a new gun laying on the ground, or dropped by an enemy, grab it and kill someone with it; it can be an enemy or an innocent person.
9mm Pistol - Found on mission Payback. Or from opening certain lockboxes.
.45cal Pistol w/ Taclight - Drug Bust: Central Sewage Outpost.
Machine Pistol - Found on Rouge cops heat levels 3 or higher.
.45cal SMG - Drug Bust: Aberdeen Sewage Outflow.
.45cal SMG w/ Taclight - Mission: The Wedding.
Pump Shotgun - Found in the trunks of cop cars if you have the upgrade unlocked.
Anti-Riot Shotgun - Found in mission Civil Discord.
Anti-Riot Shotgun w/ Taclight - Found in mission Civil Discord.
Assault Rifle - Found in mission The Funeral.
Assault Rifle w/ Taclight - Found in mission The Wedding.
Assault Rifle w/ Grenade Launcher - Found in mission The Funeral.