If you do the same guns at the same time, and one shows it and one doesn't, it can be a few factors
lower pressure guns are less likely to show it. My mag only shows vapor if it's chilly, like a night game. My phantoms always get a little vapor.
porting- if all the air is released at the end of the barrel, or more of it, it's more likely to create enough condensation to be visible. If it all escapes out of different small holes, it will be less likely to be a big enough effect
and probably other factors, efficiency for one. If two guns run the same pressure and have the same barrel, but one uses more air per shot, there's more expanding gas to create condensation
