[Charles Martel]

The number of rifles like the AR-15 in private hands has probably tripled since the assault weapons ban expired. The NRA's David Keene estimates the number now at above 3 million.

Heh, "above" 3 Million is a very rough estimate. Even the lefties over at www.liberalgunclub.com like to do group purchases of stripped lower receivers, which don't get counted as completed rifles. The number of completed ARs sold is the tip of the iceberg.

[longtermmemmory]

ok forgive this basic question, but what is the deal with “assembling” a do it yourself ar?

and what exactly is a stripped receiver?

[William Tell]

Charles Martel said: "... group purchases of stripped lower receivers, which don't get counted as completed rifles. "

If you buy a stripped receiver in Kalifornia it most certainly is a "rifle". It is the part which has the serial number and that serial number is traceable through the FFL system.