"Guns inflict Defense Down for up to 2 seconds, duration refreshes on hit; effects don't stack."

This can be accomplished by just adding Poison to a gun. Poison cuts the victim's attack and defense. No gun in the game currently inflicts Poison. (And it would be REALLY nice if there was one that did, as Gunslingers are... challenged... against the Royal Jelly boss.)

Also, far as I know, "Defense Down" only exists on the Snarbolax, where it is more of an alert that he's no longer invincible. Is there a true Defense Down effect in place anywhere else?

"...you can give that status to a large spread of enemies in a short order of time."
Well, new problem: The Alchemer series can still inflict and maintain this "Defense Down" effect to an entire swarm of foes at once with its ricochet. This is on top of the status effects it already deals. Additionally, the Antigua series' rate of fire allows it to blanket an enemy army with Defense Down as well.

And what of the magnitude of this defense decrease? Will the Proto Gun become more useful than an Autogun?

This idea only further emphasizes the two most-commonly-used guns, which weren't even focused on damage to begin with. Yes, it's helpful to the gunslingers' cause, but there's got to be an incentive to taking the less-used guns.

"Two things, the Magnus' large projectile is capable of hitting more than one target at once and the Blaster's firing rate is comparable to the Antigua's."

The Magnus' splash damage only worsens the "multi target" effect you've been trying to argue away from.

The Blaster only has a three shot clip. The Antigua has six. Presuming you stop on bullet short of unloading your full clip to allow for shield canceling, that's two shots and five shots at a time, respectively. The Antigua therefore pumps out more than twice as many bullets per burst than the Blaster, each of which now carries a defense reduction effect.

You can bring up damage resistances, but their effect is downplayed by the very defense reduction effect you are advocating, allowing your teammates' damage bonuses to more than make up for the damage lost from the gunslinger using the wrong type. You don't want to encourage using weapons the enemy is resistant to, just because it puts the defense reduction on more easily.

I have a friend who steadfastly refuses to use swords. I compared damage numbers with him and was startled by the fact that his three-shot Blaster burst dealt barely more damage than one swipe from my Calibur. With such low base damage, the defense-reduction effect gains much more value for all team members except the Gunslinger. (The difference is so pronounced that he finally caved a little while ago and posted a thread right here in the Arsenal forum asking for sword and bomb recommendations. And that's someone who LOVES gunner characters in video games a lot more than swordsmen. When I recommended the Calibur to him starting out, I could hear the serious hesitation in his response.)

This defense-debuff idea is seriously worsening the "walking status inflictions" problem by 1) being a status infliction and 2) giving the teammates a bigger bonus than the gunslinger themselves. They dealt more damage without the debuff, and a percentage-based bonus will skew numbers even further towards non-gunners' favor.

And even in the support capacity, a defense reduction effect on one enemy at a time is simply not nearly as satisfying as having a teammate who sacrifices their direct damage altogether to plant Shivermist Busters nonstop.

Now let's say the defense reduction is something crazy high, like triple damage. Bosses are now trivialized by the presence of a gunslinger.
Okay, so maybe let's not make it that high. But now all the aforementioned problems come roaring back.
Okay, let's make the damage reduction really low, like 5%. Then it's not even addressing the problems it was meant to solve.

Let's thrust a gunslinger, complete with defense reduction effect, into a situation and compare his response with that of a swordsman or bomber:
The situation: "It's a Danger Room and all your teammates just died. You've got half a health point left. Handle it!"
Swordsman: "Okay, I'll use my sword's charged attack or knockback to defeat the enemies with hit-and-run. The sword's damage is high enough for this to work."
Bomber: "Okay, I'll kite them down with explosives. I can deal full damage to several enemies at once, clearing the room at a reasonable pace."
Gunslinger: "With nobody alive to help fight, the defense reduction from my gun is heavily devalued. I'll need to kite them down, dealing small amounts of damage at a time to a single target to maximize damage output. I hope no other enemy walks in front of my designated target or else my damage output will suffer. Maybe I should put the Magnus away, the flinch effect will make my designated targets fall to the back of the group, wasting the bonus. I should put the Autogun away too, or else the enemy will catch up and finish me off."
At least he's not dealing LESS damage than before the defense reduction ability.

This solution makes everyone else's damage better and heavily encourages group play. It's great that your idea rewards team play, but it's not great that this fails to reward solo play to nearly the same extent. I want guns to be strong, and I want gunslingers to feel strong. I want NON-gunslingers to feel that gunslingers are strong. I want this problem solved. But this solution does not look like a good answer to me, as it has only served to highlight the problems with guns to begin with: namely, the gun's poor damage and increased difficulty with solo play compared to bombs and swords.

A gunslinger should be able to stand on their own without having to rely on teammates or other weapons, just as swordsmen and bombers can.

we could always implement splash damage to the guns