Values listed in brackets are for alternate (secondary) fire.
The combat shotgun is an alternative (or complement) to the regular shotgun or HSG-1 in that it serves a similar purpose, but a little differently. It has stats very similar to the shotgun except it has a much higher fire rate, a slower reload rate, and a load capacity of 6 shells instead of 8.
It is one of the three weapons that has a flashlight, the others being the 9mm and the shotgun.
As with any shotgun, it is important that you reload as often as possible. Reloading fully takes several seconds, which in a tight situation may lead to your untimely demise.
Try to avoid using over long ranges, since the spread makes it ineffectual at anything beyond medium range. Getting head shots with the 9mm will likely do more damage than a single pellet.