If you have a larger primary, (For example, my M700 is only a foot or so shorter than I am tall.) Having less shit hanging off of your body the better. Pistols or small SMG's are more 'ideal' than some shotgun. Not only that, you're limiting yourself the benefit of fire rate.
For example, I would much prefer having my USP.40 or my buddies MAC11 on me more than a shotgun, mainly because A) they're microscopic in comparison to my primary, less to worry about hanging off of me. and B) I shoot as fast as I pull the trigger or more.
Don't try and pull off some stupid tacticool bullshit or pretend like you're from Call of Duty. Use what makes real sense.