Yes, speed loaders are magazine equivalents. Sadly, you'd need at least two to be effective; technically you can load the weapon, then load a speed loader. This gives you four shots with only a single reload (with the shotgun), however you then need to remove the loader from the weapon manually, then load each unit again. With two loaders, reloading or using the action key will swap between the loaders regardless if they're full or not removing the need to manually remove a loader from the weapon.