Some details of it

I found info related to this skill probably it may help to understand how it does work. For now.

Think of Frenzy as the "Final Hit" for gunners. What does it do? Well first off, you cannot teleport with it, you cannot drink potions during the duration, and when the skill ends you will be left with an empty clip? The good part? Well you will have unlimited ammo (you can even use this on an empty clip right away). Sounds good, but if it was only that it would be a lackluster skill. The special part about Frenzy? All hits will become critical hits, meaning you will score crit after crit after crit. Which is very useful if used properly.

Efects:

*Reset All Skill currently in cool down
*Attack speed increase
*All normal attacks will become critical hit, regardless of the enemy's protection.
*Infinite Ammo
*When under the influence of Frenzy, Bullet Storm can hit an unlimited amount of enemies within range.
*135 AP (may vary acording with the current stated info)

Info by VermaWyrda Mabinogi NA forums

But can't Final Hit already be used with Dual Guns? I don't really think comparing to Final Hit is accurate purely based on that it is a buff skill.

Also, there is a method to make the clip not be completely empty after use, but I'm not sure if that would be considered an abusable glitch since being hit after using aftershock resets the cooldown and that's a confirmed intended feature.

Shockwave resetting was never in doubt as a glitch; it behaving that way is in the description and triggering its reset is even a training requirement. The glitch on the other hand counter to what the description of the skill says so it's clearly a glitch, even though some would consider it a clever circumvention.

It's also worth noting that "Defeat X enemies or more while Way of the Gun skill in effect" can repeat itself multiple times during a single usage.