It really depends where in Littleton you are located. Some indoor options are:

http://www.theshootistpistolrange.com/ (poor ventilation, but always had good luck getting a lane)

http://www.blucoreshootingcenter.com/ (good ventilation, good lanes, can get very busy)

http://centennialgunclub.com/shooting-range/ (new facility is very nice, great ventilation)