I'm looking to get a water gun that could be filled up with slime and shoot it a few metres. Obviously the viscosity will make it perform way below spec, but as long as a gunge fight can be had that's fine

I was looking at the Super Soaker Flash Flood, here... especially the 'shotgun' mode looks good, but it seems to have been relaunched as a much inferior squirter with no air pump.

If I could get one, reckon it'll work with slime, thin custard, sort of thing?

https://m.youtube.com/watch?v=hCnpBvyiwnw

You may want to google "Stream Machine." These are some of the simplest water guns out there (almost literally just a piston) but they can shoot well over 50 feet away. And I am sure they would be much easier to clean out than a regular water gun.

We use gunge blasters which can shoot thick gunge, custard and all sorts of stuff. You can get them from Messy Supplies here: http://messysupplies.com/index.php?route=product/category&path=251

probably work well with cream too