I think your asking on the wrong website, but the combo's vary from state to state. People in this forum say that it is illegal to put a mag extension on a base M4 (fixed pistol grip stock) in California when according to the California Department of Justice it is not.
If you happen to live in California then it would be illegal to have a pistol grip with a collapsable stock. Whichever state you live in try this but change the state.
Google - California Department of Justice
Which brings "Office of the attorney General"
( http://ag.ca.gov/ )
Click on "Publications" - "Firearms" - "California firearms law (current year)"
On Page "13" item "6"
"A semiautomatic shotgun that has both of the following:
(A) A folding or telescoping stock
(B) A pistol grip that protrudes conspicuously beneath the action of the weapon, thumbhole stock, or vertical handgrip.
(7) A semiautomatic shotgun that has the ability to accept a detachable magazine.
(8) Any shotgun with a revolving cylinder.
The following definitions shall apply relative to defining assault weapons:
(1) "Magazine" shall mean any ammunition feeding device.
(2)"Capacity to accept more than ten rounds" shall mean capable of accomodating more than ten rounds, but shall not be construed to include a feeding device that has been permanently altered so that it cannot accomodate more than ten rounds.
G/L in your search.