I agree, but I see a more simple option. When you go into your trunk (F4), either you recieve an AR15 or give an option. Because there are shootouts when you encounter multiple assailants and they have assault weapons. A shotgun doesn't have the range and the handgun isn't enough power when you have hard targets. I was looking the through the code today to see if change the Shotgun to an AR15 was a simple task or not. But I can't locate the scripts that allow you to add the weapons.
Would be nice to have though. But they can keep doing what they are doing. And should they decide to bring out an official game, they have my $60. Even if its not GTA, I think they have they have the concepts and clearly there have been mark-offs of GTA since its release.