re: AR-15 for dummies.

If you look at the parts lists in that photo, #1 on the right side is the upper receiver and #2 on the left side is the lower receiver.

The two receiver halves line up together and join with two push pins.

So to answer your question about the difficulty in assembling a gun, it depends on how finished the upper/lower is.


With a complete upper, such as:




And a complete lower, such as:




You just line up the two pins, push them closed and you're done.



Turning a stripped lower such as:


into a complete lower is relatively simple but does require more tools and planning. Same goes for turning a stripped upper into a complete upper.





Note that when buying parts from retailers, the "gun" that requires a background check is the stripped lower. Whether it's a complete rifle, a complete lower, or a stripped lower, if it has the lower receiver with holes in it for a fire control group it's "a gun" whereas everything else (i.e. complete uppers) are just parts and can be shipped to your house.