If you look at the weapon itself, you’ll see subtle differences between the parts. Grips on shotguns might have the most subtle differences of all of these, so that’s a bad example to start. Go to a weapon vending machine and look for two weapons from the same manufacturer (say Bandit SMGs). What’s different about them? Barrel? Stock? Grip? Sight?
There are some very well laid out infographics about this that someone should be posting here shortly (I have them saved on my desktop, but they are hosted online somewhere) that really open your eyes to this.