Mods tab in the Store.

Mods is a tab in the Store that has three Attachments available for purchase:  High Explosive, Extended Clip, and Recoil Pad.

Both the Extended Clip and Recoil Pad Mods cost the same for each firearm, $900 and $1200 respectively. The High Explosive Mod costs $1500 and is only available for the M67 (Grenade).

ModsHigh Explosive Extended Clip Recoil Pad
AmmoExplosive Ammo Incendiary Ammo
TacticalAmmo Counter Laser Sight Flashlight Suppressor
OpticsIron Sight Reflex Sight Holographic Sight Zoom x6 Scope ACOG Scope Zoom x4 Scope