The bubble gun is not the problem. No more than queue jumpers, one family member "reserving" spots for the other 5 who have obviously gone and done a FP on Screamin' or something in the meantime, the people who invade your personal space and bump into you / your backpack 5 times a minute, or for that matter, those who hug all the plush toys and try on all the hats in the World of
Disney Store whilst sweating like a pig on a 90 degree day (without any intention of purchasing any of those things). I fully understand that some people dislike kids with bubble guns, and I fully get it and respect it. And that's my point. Respect. Disneyland is a part of life, and part of life involves teaching your kids right and wrong. I don't teach my child Disney etiquette, as much as I have taught her "social etiquette". Bad / selfish behaviour is inappropriate anywhere, not just in a ride line at Disney.
Here is my thoughts on the bubble gun thing, and my DD9 knows this well, and fully respects the rights of others. She doesn't use the bubble gun:
1) When the line is indoors, because there tends to be tiles, etc. and people may slip on the soapy residue
2) When the line is outdoors, and there is no "breathing space" between guests
3) She never points the gun at anybody, especially at faces
4) She releases the bubbles slowly, and not just continuously in a great shower of suds
5) She stops if the kids around her start to get too hyper. It happened once when there was a young kid, who was already quite aggressive, and was trying to pull out all the poles that held the chains for the lines. The boy started screaming at the bubbles, so she stopped immediately.
6) She stops immediately if we ask her to, or if anybody else asks her to stop (which has never happened)
7) When the wind is likely to blow the bubbles in an unintended direction (ie towards other guests)
There are obviously lines where the bubble gun is not appropriate (eg. Storybook canal, Alice) EVEN if the line is outdoors. But there are also lines with long waits, where there is either open space, or a place for kids to shoot bubbles that don't hit other guests, such as Toy Story Mania (especially as the queue gets to the bit facing the Boardwalk games), Buzz Lighter (the queue is against the side of a cabbage / rosemary garden).
Teaching kids discipline and consideration if the key, because bubble gun or not, they have the capability of annoying those around them.