[Paul_Denton]

I guess what I'm trying to ask is: how and why do we decide which arms are and aren't allowed under the 2nd Amendment?

Well you can thank the left for that. The 2nd Amendment has been infringed upon since the 1930s.

[steve-b]

The 2nd Amendment has been infringed upon since the 1930s.

Actually, major infringements of the 2nd Amendment go back to the post-Reconstruction South, for obvious reasons (ever try to wash blood stains out of white sheets?)