Going more on what Nutterbutter was talking about, you should be able to make modifications to your weapons such as: chopping off barrels, chopping off the stocks, etc. This would make concealing weapons much more realistic than the "GTA: AK47 and rocketlauncher in the butt" method of hiding weapons in games.
Weapon modding would go hand in hand with a gunsmith skill. If your skill is not high enough you might end up breaking your gun or just making a terrible cut so it looks worse.