I believe a large part is do to the FSC30. I just put one on my DPMS 16'' 308 and its night and day difference from the stock birdcage. PTR91 (HK91) has always been known for harsh recoil.
This is very true. The PTR-91 has been my MBR for years, and I can tell you, you learn to live with and love shooting .308. Coming from a shotgun background prior to buying my first high power rifle (my 91), I was used to dealing with recoil. Now, I can shoot .308 all day with no problems. I would like to see a picture of the inside of that stock adapter on the PTR-91 in the video. I am betting it doesn't have a buffer in it. A buffer, be it factory HK, or aftermarket, makes a world of difference on the G3 style rifles.