Temple of Ranma's Sailor Senshi Seifuku

The gun in question is in the picture below. I was wondering if there was an existing gun that looks even remotely like it, so I can use that as a base and build on it to help describing what the gun looks like to anyone who hasn't seen the gun I'm trying to describe.

That looks to me like it would be some variant of either a magnetic accelerator, or a hand held railgun.

If you mean real life... no human could hold a gun that size or survive the probable kickback.

If you mean in fiction...

There was the one scene from Mahoromatic with a gun somewhat similar.

Ceres Victoria had a healthy appreciation for extremely large guns, but hers did not look like that. Kicknamed Harkonen, several variants exist between anime, OAV, and manga.
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A Starfleet Type 3a First Contact era Pulse Rifle looks like that, but grey and on a smaller scale. It is a magnetic accelerator.

There was that huge heavy gun in Aliens... the one that needed a second person to help feed the ammo. That was more of a gatling cannon than a gun.

Fiction or real-life works. If it can be referenced, mission accomplished. One of the problems I have, aside from my limited knowledge of guns and their parts, is that I didn't want to assign it a type with too many variations on its design. For instance, my first guess was that it might be a railgun, because I've seen those with split barrels (the gun in the image opens vertically), but looking at all of the designs (concept or otherwise) made me doubtful that just calling it a railgun would be a sufficient enough foundation for describing it. And then there's just so much gun... Like, if I had to guess, behind the grip and trigger would be a stock, so I have no idea what to call the large section between those and the barrel. The chamber? And I'm not sure what to call the part below that and the grip, with the handle that offers extra leverage.

Closest i can think of is probably some of the guns used by Starcraft series Ghost units. And that´s still not even remotely close.

Hmm, and now i can´t even find the pic i´m thinking of, because the common weapon they´re seen with is smaller and more realistic, and even the one i was thinking of is probably not even half the size of your pic.

My first suggestion is that you consider the picture representation to be oversized, probably as an emphasis thing, as that kind of sized weapon, you might see it on a tank or a ship, but even if you halve the size it´s still beyond absurdly large. And even if you do that, you can still start off by calling it just that, "absurdly large".

If not, the user is definitely not limited to anything resembling human strength.

Like, if I had to guess, behind the grip and trigger would be a stock

If you look closely at the picture, those are the parts that end behind her back. At least one part each of what is seen behind her shoulder and ass are the rear ends of the upper and lower part of the "gun" handle.

Hmm... An H-shaped frame with the trigger and grip on the horizontal line of the "H" and the barrel mounted between the forward prongs.
Insufficient, but at least hopefully understandable and a start.

Could be sometype of engery weapon also. Then again I would call it a engery support weapon that is about six feet long and some how the little girl can swing it around and fire it with easy skill. If nothing else.....its your standard big fragging gun with magical lighting and leave it at that.

It occurs to me that while that is much bigger and angular... it very closely resembles at least in the handle section the black market weapon being sold in the movie called Fifth Element. Called the ZF1

Zorg Presents the ZF1
- https://www.youtube.com/watch?v=7jVsQToSfag

I don't think you can use that description... but I always liked that gun...

I would call it a engery support weapon that is about six feet long

The weapon is about two and almost a half times as long(27cm onscreen) as the girl(11cm onscreen).

So unless she´s a lot less than 3 feet tall, it´s much longer than 6 feet.

I wish I'd found this earlier, but here's a better view of the gun. Well, two versions of it, anyway.

Is that by any chance called Blackrain?

The next generation model of Railgun. The color has changed from white to a more covert black.

If not, is it by chance Linear Railgun?

The ultimate Railgun made possible thanks to a collaboration using cutting-edge technologies from both Lastation and Planeptune.

The above are the descriptions from the Hyper Dimensional Neptunia Weapons Wiki...

Nope. Those are some of the standard guns. When Uni is in HDD mode, they are all replaced by this one.