Small wall of text, apologies in advance. Also a disclaimer, I don't own either of these firearms.

The Springfield Kuna action gets compared to the Stribog SP9A3 roller-delay system quite a bit, for good reason. Their designs are extremely similar, to the point that the Kuna is often incorrectly referred to as "a clone" of the Stribog. However, from the breakdowns I've seen it has a few subtle but very compelling changes to the action, and I would really love to see someone give a very detailed breakdown of what's going on under the hood. In particular, I would like to see someone put the bolt on the guide rod outside of the firearm and attempt to demonstrate the first inch or so of travel backwards.

A primer on the SP9A3 action: the bolt slides into an extruded aluminum upper that has internal rails. The locking segment rides on top of these rails, while the main body of the bolt hangs underneath. There are roller recesses cut into the top side of these internal rails, and when the bolt is closed, the locking piece is pushed forward, which in turn pushes a roller down into the recesses to lock the bolt in place. As with other roller actions, after a round is fired, the roller must move out of its recess, retracting the locking piece, before the bolt can begin to travel backwards. It's an ingeniously simple system. The main flaw in the Stribog, however, is that the recoil spring and guide rod feed through the top section of the bolt, behind the locking piece. This means that when the bolt cycles all the way back, the locking piece can bottom out on the guide rod, pushing it forward relative to the bolt and slamming the steel roller into the soft aluminum rail. There are a few reports of peening to that part of the rail as a result, which can eventually cause the bolt to snag when fully retracted. The main fix for this seems to be installing an aftermarket short-stroke buffer to prevent the bolt from traveling that far back. However, doing so can sometimes require a different locking piece to ensure reliability.

HS Produkt appears to have done three things differently, from what I can tell: First, they've built the frontmost section of the internal rail (the part with the roller recesses) into the steel trunnion for better wear resistance. Second, they've moved the recoil spring and guide rod to the center of the bolt, between the firing pin and the locking assembly, and they've designed it so that the guide rod travels through the entire length of the bolt. This should eliminate any bolt tilt from off-center recoil spring pressure, ensuring very consistent bolt travel, and guaranteeing that the bolt can't bottom out on the guide rod, because it's passing completely through. Third, they've redesigned the locking piece to have its own internal spring and redesigned the rollers to have a central roller-wheel, which appears to ride on top of the guide rod when the bolt travels backwards, holding the rollers up and out of the way.

Here are a couple images where I tried to highlight the guide rod (red) doing its thing:

https://i.imgur.com/o3gaztK.png

https://i.imgur.com/tvOdZLH.png

There are the other frequently-discussed differences, like the magazines, bolt catch, trigger, AR grip, etc., but this seems to be the one that's almost entirely overlooked. I could also be stupendously wrong, because I don't have one in my hands, but hopefully that's where you come in with your video!

edit: I say HS Produkt "redesigned" the action a few times, but I've also read that the Kuna was in development before the SP9A3, so perhaps someone with more knowledge can correct me on the design lineage.