In GTA Online, weapon attachments unlock as you increase your rank. For example, you unlock the Advanced Rifle at Level 70 and the Extended Clip for the Advanced Rifle at Level 71.
DLC weapons are located next to the original ones, listed in the appearances order (i.e., the Gusenberg Sweeper will be before the Bullpup Rifle). However, the Sawed-off Shotgun can be placed here first if the Assault SMG is available.
Sometimes, guns dropped on the ground by NPCs will randomly discharge, as seen here.
As demonstrated on the gameplay video, weapon system on the enhanced edition of GTA V now introduces the ability to throw grenades while holding a weapon. It is done by either pressing Left on the D-pad, or by swiping the Dualshock 4 touchpad upwards. On the PC version, it can be done by pressing "G" whilst aiming. It is available in both first and third person modes.
Most of the game's weapons feature reload animations unrealistic towards real firearms (for example, shotguns are always pumped after reloading, even when not empty, which while possible would needlessly eject a loaded shell.) However, this was likely done to allow for speedier reloading in game.