Marker is a politically correct term, it's origin goes back to the original purpose of paintball guns.
They were designed for forestry and farming and fired 'paintball marking pellets' with oil based paint to 'permanently' mark trees and livestock, eg picking trees for felling
America (like Victorian England) is a land of contradiction - support the services, freedom and rights to be armed, but having any related skills or interest makes you a nutter
Add shootings, southern hicks & conservative parental fears, and the desire to get paintball televised with lots of corporate finance and people tried to distance paintball from the woods, ban people from wearing camouflage and have an imaginary reputation of a 'pure non gun sport'
This was less of an issue in the UK until things such as Michael Ryan
Every now and then the perception goes around in circles with some people trying to seperate different styles of paintball - instead of reinforcing their preferred style this fractures the paintball community
Use marker as a term if you wish, it's handy sometimes to skip a knee jerk reaction to the word gun, however if you are in a situation where that may matter then do bear in mind the impact of the other party not understanding what you are on about and then thinking you have lied about the gun they discover.
(Eg should you ever be in the situation of having your car searched by the police and they ask if you have anything they should be aware of that if you say you have markers, they may not be impressed with finding guns)
Guns are legal and in the eyes of the law we use paintball guns