The priest already has a small gold plated or bronze instrument which he can dip in a vessel and use to fling droplets of holy water on crowds. I can’t remember the name of it but it looks like a little bronze ball on the end of a stick. They do this on special occasions. I remember it being done somewhere around Easter to the big crowd at Mass.
I’ve gotten hit by a droplet or two from that instrument. It doesn’t stand out from any of the other paraphernalia that they use at mass other than that it can be used to fling holy water.
Father Ripperger has a video in which he recommends ways to combat demons (Spiritual Warfare?? - anyway, something like that . . .) which include things like eating and drinking different blessed oils and salts. If memory serves, I think he recommended consuming holy water as well. Holy water can also be sprinkled around one’s dwelling and other belongings. I think a misting device, more like the mist from a perfume bottle might be more in order, although I suppose you could just use your fingertips.
All that said, I tend to agree with the other posters on the issue of irreverence.
If there were to be something akin to a holy water gun to be developed, I would think it would have to be made of something other then plastic, at least initially. It would have to be a material and a design which dispensed with the comedic aspect. I would think a good start might be something similar to a gun with both a spray and a stream setting, like certain sorts of Windex-type bottles, but made of bronze with both gold plating inside and out.
After that had become more common, an argument could then be made for making cheap pretty ornamental varieties of plain bronze, brass, glass, aluminum (at one time, not long after it was discovered, aluminum was one of the world’s rarest and most precious metals) or plastic for use in poorer regions and for use in and around the home. Rosaries, after all, are made of a variety of materials, some of which are plastic.
It should probably have a ‘trigger’ which could include up to three fingers for added pressure and range because it could then reach the deep interior of large crowds, or at least that could be one of its initial uses.
I doubt that that idea will go very far because masses seem to be fairly traditional and because mass is, after all, a Sacrifice. I don’t really see anything inherently sinful about the idea, it’s just not very traditional, and at this time, it’s still irreverent (but it needn’t continue to be that way). I don’t think God would think there is anything inherently sinful and wrong with a finger pressure hydraulic device other than the jarringly comedic aspect of cheap plastic and explicitly gun-type shapes. It ought to blend with the Sacrificial aspect of the mass and not stand out. I think I’ve denoted a vague sort of ‘path’ (gold plate → bronze → glass → ornamental [perhaps somewhat sparkly] plastic [like a rosary]) for it’s adoption, but I doubt anything will come of it.
