One of the most important aspects, is the fact that it's purely cosmetic. Functionally, there are only a handful of firearm designs. Bolt action, Direct impingement, gas piston, single shot, etc. Functionally, they are all the same, with different minor variations. Now if you transfer that over to something else...Let's say a car. Ford, Chevrolet, etc. all make coupes and sedans, but nobody can lay claim to the coupe or sedan body style, and likewise the operating mechanisms of the AR-15 is nothing that hasn't been around 100 years now, so there's no patent on it. What Colt had (bought from Stoner, as already covered) was a TRADEMARK. They owned the rights to the AR-15 name. That means that any other manufacturer can build the same gun, they just can't use the same name. That's why Stag uses the 15A, 15R3, 15L, 15L2. Remington uses R-15. Etcetera. Again, like in cars, ford designed the first "pony car" with the Mustang...It was a compact chassis, rear wheel drive coupe with a sporty V8. Then Pontiac, Dodge, Chevrolet, Plymouth, etc. all made similar cars, and the "pony car" wars began. But Ford couldn't sue those others for using the same formula, they just couldn't call their cars mustangs. That's why Remington, Ruger, Savage, Mossberg, Leica, etc. can all make a bolt action rifle and they all look alike, and that's why Stag, Ruger, Remington, Rock River, Bushmaster, DPMS can all make AR-15 style rifles, and they all have their own names. The only difference is, they all used the same design for modularity for upgrades and replacements, so you could piece together whatever parts you wanted, thus creating the ability to put a Bushmaster upper with a DPMS lower and Magpul stock and MOE grips and Troy mags and...you get the idea.