As I understand it, (library pictures support this too) most of the Vietnam full auto "chop mods" simply cut back the FE, the idea apparently to make it boom like a .50 cal, so I have read.
It really would be very interesting to hear from veterans who either carried out the work or indeed used them in anger.
It's such a little know part of the inch pattern story, great to finally drag it into the light and get some hard facts.