= new reply since forum marked as read
they have spring mechanisms in the stock that absorb most of the energy to operate the bolt action to eject the cartridge and re-load the next round.
...or the impact of the bullet with body armor?
The kick hitting the sternum doesn't make much sense.
Of course, neither does adding full-contact tactile feedback to a video game! 
I first qualified Marksman with the M-14, which was basically an updated M1 Garand, with all the kick you expect from an old school rifle. Given my total lack of any experience with shooting, that was a minor miracle. But they "adjusted" my final score because it was based partially on a section at 500 yards, but the actual firing range was at 500 meters. Guess they didn't want MC privates running around without a shooting medal. 
After bootcamp, we were issued M16s, which by comparison felt like BB guns. 
On subsequent qualifications I shot expert with the M16. It's a pretty easy gun to shoot.