The first guns were made from cast bronze, not good steel. To have them made from steel, but be limited to nobles would suggest a heavily bloody history of gun fighting after which guns were outlawed and destroyed (Think england, only much more bloody). This would make a large underground market of hidden guns. These guns would be old and unreliable. Only the newer guns made for the nobility would be reliable. And, of course, due to nobility being able to say "I don't care, just make it the way I command" many of these guns are likely to be silly, with barrels cast in the shape of dragons heads, Ridiculously big or long (to account for an inadequacy), made with precious metals and jewels, and otherwise be vastly eye-catching and recognizable. For them, it's part of thier wardrobe.
An interesting note about early forms of gunpowder. When first mixed with early techniques, gun powder would settle, with the charcoal, sulfur and saltpewter seperating. After marching for a hundred miles, the soildiers would try to fire their guns and nothing happens. It wasn't untill people discovered wet mixing that gunpowder would fire reliably. You can let your players think they're being all clever, making their own gunpowder and avoiding the 100 gp a shot cost, just give thier home made gunpowder a chance of firing 19 or less on a d 20 to fire, minus 1 for every 5 miles marched. March 50 miles, your gunpowder hads a 50/50 chance of working. The powder makers should probally know how to mix theirs wet, making reliability worth the cost.