Sure, it quite legal to own a Busmaster or any other M4 semi auto variant with 30 round mags as long as they were purchased before 2001 and the weapon was registered with CA DOJ.
Ha, ha, that's funny - you can buy that rig here in Oregon, today - without registering it with any DOJ. I purchased mine a couple of months ago.
I also have a Ruger Mini 14 Bullpup with a 90 rd drum mag, can you purchase one of those in CA?
Thanks you and congratulations on your military service and for your current service; protecting our borders!!
Good man!!