Actually, it felt different for me when I bought my first firearm as a US citizen than as a legal resident alien.
Now, one less piece of ID is needed, as well as having to no longer produce proof of residence papers (like a phone bill with my address on it) , and one less line to fill out on the form (green card number).
Actually, I was more suprised that I could one a gun and not be a citizen. Since there were/are quite a few other things non-citizens can't have that I know about, I figured it applied to guns too. If you''re told no, often enough when asking questions about being able to have or do something, your sort of stop asking and just assume no.