Here's a few quotes. Some given to me, some I've found:
Heir to the Empire, p182
All the Star Destroyer's batteries seemed to open up at once, and Luke suddenly found himself trying to dodge a virtual sandstorm of laser fire.
Clear enough. If all the weapons seemed to open up, then it's a good bet that most, if not all, of them did.
Agents of Chaos 1: Hero's Trial, page 181
Biding its time, the cruiser waited until the Yuuzhan Vong warship paused to repower, then it opened fire with all guns. Still stronger laser beams sliced through the night, some to be swallowed by gravitic anomalies, while others chipped away at the enemy ship's Yorik coral hull.
"All guns." Simple enough.
I, Jedi, p353
As the aft came around, the Invidious hit it with everything. The Harmzuay's aft looked like a black hole sucking in every green energy shaft the Invidious' guns spat out.
That might be from page 352, since I edited out a bunch of the multipage quote, and I'm currently letting someone borrow my copy of the book. Anyway, the ISD hit the Harmzuay "with everything" -- all guns. Accuracy was high, too.
Earlier in the quote HTL usage is described, and continued use is implied here.
The Krytos Trap, p324
The turbolaser batteries in the bow started firing at the skyhook as they came into range, then the broadside assault shifted to other weapons as the ship slid past. The verdant laser-bolts came so fast and so close that whole sheets of energy seemed to pulse from the Lusankya to the skyhook. In seconds what had once been an elegant disk with an Ithorian jungle paradise at its heart became a melted demilune with a forest fire crashing into the mountain district's towers.
All weapons didn't fire at once because they weren't all available due to angles. Still, the weapons that did fire were very close together and the assault only took "seconds" even though several of these salvos were fired.
The Krytos Trap, p325-26
The Lusankya continued to pour fire into the planetary shields while what little ground fire that came up at it splashed harmlessly on it shields. The midship and stern guns fought to keep the hole in the lower shields open while the bow guns blasted away at the upper shield. The ship's assault sent waves of Rodian green energy skittering along the underside of the shields. The shields held at first, then began to erode, and finally collapsed.
Guns from each section of the ship, and mentions of the "waves of ... energy" again.
The Bacta War, p307
Over a hundred starboard ion cannons fired back at the Freedom in a display so massive it appeared as if sheets of blue energy had erupted from the Lusankya's side.
Only a bit more than a hundred? Aww...
The Bacta War, p318
"Guns, let him have everything!"
"As ordered, Captain."
The Lusankya fired its starboard weapons at the Imperial Star Destroyer, mauling it mercilessly. Turbolasers crushed the shields while ion cannon beams skittered over the Freedom's hull. Concussion missiles peppered the smaller ship, opening huge hulls in the hull. Explosions wracked the Freedom, spraying debris in all directions.
Look, "everything" means
everything. All guns -- turbolaser and ion cannons -- and even missiles.
Well, that's what I found after a quick search of the quotes I have stored on my computer.