What part actually eats the recoil impulse from the upper receiver and transfers it to the lower receiver and stock? Is it the takedown pins or any of the contact surfaces between the receiver halves?
Sort of all of the above.
Take down pins do the majority. If the upper and lower are a good fit, then the rear of the upper should connect with the upper of the lower. But remember, a lot is also taken up with the buffer tube and spring.
Check out some high speed camera video. It is way cool
The take down and pivot pin take most of it.
I’m thinking of the first impulse right at firing, before the bolt unlocks. I’ve heard before that the pins do the job, but the surface on the rear of the upper is quite large and seems suitable. I guess the reality of tolerances mean you cant really fit the pin holes together and have the receivers fit perfectly as well.
Thank you for posting that. Man, what an amazing animation!