I haven’t followed this thread until now. I’ve got a few extensions off of eBay. Just looked at m2 and m4 stuff on there. Seems to be a few options. I put a carlsons extension on my wife’s 11-87 20g youth. I see Carlson listed for the M2 right now but unless I missed it, none for the m4 at the moment.
Any of them. They’re all more gun than is probably necessary for home defense, unless you live in a really dangerous neighborhood and keep lots of cash and/or drugs around.