you could totally use them for outdoors, indoors is another issue.
Some race guns are built for higher power which exceed the indoor fps limits, so they can't be used.
Other issue is purpose built race guns decked out to the nines tend to have a lot of things on them making them extremely hard to skirmish. A lot of accessories will get caught on gear easier, and its next to impossible to find a holster that will hold the gun properly without either damaging it, or make it really easy to lose when you're on the run.
I know MadMax has a pistol which he uses for CAPS, but is only moderately modified so he can use it for skirmishes too, while others like myself have pistols which are strictly built for only target shooting.