[Arrissen]

This all really depends on the unit, the setting and the current risk level as to who carries what and these days alot more money has been thrown at front line units. This means that yeah, some pilots will carry carbines or sub-carbines or an SMG in addition to their issue sidearm i.e. the US Army's 160th SOAR do this. These guys support SF units and are trained and authorised to do so in case they get shot down and have to put their SERE training into effect. Most modern troops on bases deployed overseas whether it's a main base or forward operating base (FOB) are required to carry their issued weapons with them at all times for security reasons. And that means for both base security reasons and for their own personal security. Besides, they have signed for their weapon(s) and will be held responsible for them. This is why you see guys standing in the chow line or whatever maybe dressed lightly but still armed, and if they're not for some good reason i.e. they're in hostpital, then their weapons will be locked away and re-issued to them later.

The more funding a unit has the more availability there will be for things (some would say luxuries) like sidearms and shotguns. For example, SF units get the most funding so an operator will be issued, as standard, a primary weapon and a secondry weapon (a sidearm). During close quarters battle (CQB) they are trained to immediately quick transition to their secondry weapon if their primary weapon fails to fire in order to maintain a good volume of fire. Remember, they work in small teams and so in a firefight they need as many rounds going down range as possible. They talk to eachother and state what they're their doing so their shooting partner/ team will know what's happening so as to maintain good spatial awareness. They will then take cover if or when they can and reload. When operating in built up areas they will carry at least one shotgun per squad, but this is more for breaching hard points i.e. locked doors than for actual fighting, although they can get used for this if necessary and of course as stated, they're good for hunting. Marine units are moving toward this same sort of system but historically have received the least funding and have therefore been required to do more with less, so not everyone will say be issued a sidearm.

Shotguns may be in scarce supply. MP's and security units routinely employ the use of shotguns and may issue them as a primary weapon but usually supported by someone else carrying a rifle, carbine or SMG, with both people working/ moving together as a two-man team. As for non-issue weapons being used, this would be more common in T2K IMO given the dire situation, rather than in a real world setting, but it would once again depend on the unit, standing orders on such things and the current level of discipline. Modern well-trained units use task specific equipment in order to maximise their chances of sucessfully completing their mission. It's not so much about what they'd like to carry.

As a player I always found that you could carry whatever you liked within reason, but still prefered to maintain my original issue weapon(s) out of a sense of personal pride and satisfaction, and also in order to maintain compatibility of ammunition type(s) with my team mates, so as to increase our overall battlefield effectiveness as a team. I hope this helps.