The operating system of the HK doesn't reduce any recoil at all, so you have that massive chunk of metal in the bolt slamming backwards and the stock butt pad isn't much. It's the nature of the G3s too that the ergonomics are not the best.
Don't get me wrong, I love my PTR91, it's an extremely well built copy and I enjoy shooting it a lot, but it hits the shoulder harder than any semi I own between the recoil and the ergonomics.
I purchased a PTR end of October and I can second that it has a harder recoil than my Garands, FN's, or M1a's. And my buttstock is too short. Lots of creep in the trigger, hard to cock the action, and the rear sight is a pain to adjust when zeroing the rifle. I shot my PTR after a "Garand" match, this Saturday, and I was flinching more after 10 rounds with it, than after 58 rounds with my match 30-06 Garand.
The PTR91 excels in ease of manufacture and ease of maintenance. It is first and foremost a battle rifle. It would be a great rifle to hand marginally trained troops, whose life expectancy is NTE 9 months. It is simple to operate, it will go bang.
The absolute best semi auto rifle for iron sight and scope use is the M1a. A match accuratized M1a is MOA accurate all the way to 1000 yards. You can put in a 5, 10, or 20 round mag, makes it easy to carry. The rear sights are the best ever put on a battle rifle. The trigger is easy to tune, lots of instructions on the web. The scope position is higher than a bolt gun, but there are mounts that allow you to see your irons, if you want.
The Remington semi autos are functional, cheap, but not that accurate. But they are popular for hunting in heavy brush where a 100 yard shot is a long shot.