It seems like the only reason some one would want one of these guns is for show, or to only use the charge on these weapons, which is still fairly powerful regardless of it's "invincitinks" (http://forums.spiralknights.com/en/node/118717) but you'd need good RNG to actually deal damage with it.
Since in these guns' basic attacks have melee incorporated in them and you have to launch yourself into enemies to hit them, it acts more like a sword,
and generally with basic attacks per hit, guns deal less damage than swords. This 2-shot gun deals the same, if not less damage than the first two swings of a Winmillion (If they were both 4*, or if the Winmillion ever.. got...... a 5*...)
The basic attacks of the Tortodrone event guns deal less damage than they should. Even though they deal pretty decent knockback, it's still not worth all that much to use their basic attacks, and you might as well grab a different gun if you're not using it for it's charge.
I know I didn't include much at all about different damage types and that it would probably be a bit better if you were using a specific damage type towards the corresponding enemies, but.... why not make a different gun instead, still?
The charge can deal plenty of damage if you aimed it right and that good ol' RNG landed shots together on enemies, but just playing with "chance" most of the time with a gun that lacks everything else, really doesn't seem all that worth it.
It's a gun. Just like Winmillion cannot be allowed to have competitive bullets, Tortogun cannot be allowed to have a competetive melee attack.
You use it to push enemies around, and it's not even all that weak.