Is there any way to convert modern rifles to full auto?

Disregarding any legal issues (class three, manufacturers licenses, etc) is there any way to convert some modern post Assault Weapons Ban to select fire? I know some imported guns are virtually impossible to do so with (FN Scar), but what about the Bushmaster ACR and other American short stroke systems? I did some research, and found nothing for spare parts to do so, or even the automatic operation for the weapon; I only found some AR-15 parts which I'm sure wouldn't work. Would a AR-15 auto sear work in some cases if the bolt is modified? Is there any possible way to convert some weapons to full auto?