The first time Bucky catches a shield, he is 7 and Steve has just thrown him a garbage lid. "Gladiators?" Steve lisps (he has just lost his first tooth and he's very proud of it). Bucky grins and off they go. It's not a very good shield but then, it doesn't need to be. Even at his most competitive, Steve would never hurt Bucky - at 7, Bucky doesn't need a shield because the only person that could hurt him never would. He doesn't understand quite yet that sometimes shields are people and that one day he will be too.
The last time Bucky catches the shield as *Bucky* is a cold day in 1944, the day before they go after a train in the Alps. He's helping Steve practice because he knows now that shields are often peple. He is Steve's and Steve is his and the best way to shield Steve right now is to make sure the overzealous moron keeps in top top shape using the only thing that regularly keeps bullets out of his brain. He's helping Steve practice because he doesn't know how to be anything other than Steve's shield - his first shield, a lesson he learned at 12 the first time Steve got into a fight, a real fight (a dangerous fight) and Bucky took a punch that wasn't meant for him and threw more back in retaliation, in protection (and he knew, even at 12, he would take punches for Steve for the rest of his life, and that's how Bucky Barnes learns that he is and always will be Steve Rogers' shield). Steve is all muscles and power now and Bucky is so glad to see a body that matches his spirit that it *hurts* but Bucky doesn't know what his purpose in the world is anymore because he is a shield but Steve doesn't need shielding. And even if he did, this shiny, colorful thing is a much better choice. It, at least, is not broken. The last time Bucky catches the shield he knows in his bones that even if Steve never needs him again, he will always belong to Steve, the same way this ridiculous frisbee does. And the next day, on a train in the Alps, when Bucky takes another hit that isn't meant for him, all that goes through his mind as he falls is that he's grateful he could shield Steve's six one last time.
The first time Bucky catches the shield, he is 70 some odd years removed from a train in the Alps. He's held it since then, of course, but he doesn't like to count the times the Soldier touched the shield. It's his starkest reminder of how completely HYDRA warped him - the Soldier used the shield as a weapon against Steve and, well, in his right mind there's not a force on the planet or beyond it that would make Bucky do the same. But Steve is gone now and the weapons are not and he is back to not knowing how to exist in the world anymore. He watches Sam practice and he knows in his bones that he'll follow Sam (Steve chose Sam. And when Bucky is lost, he can always follow Steve because Steve never choose wrong. Steve is his North Star and Steve pointed to Sam. So Bucky will go.). Sam flings the shield and Bucky catches it and, for a moment, he is 7 and catching a garbage lid and then it's 1944 and he's helping Steve train and he knows, for one bright, blinding moment, exactly who he is. The shield sits quietly against his arm, comfortable, like it remembers him. Maybe it does. The last time it rested in his hands, they were both the same thing: Steve Rogers' protection. As long as Bucky Barnes breathes, he will be Steve's shield, and this disc of metal will be so even longer. He readjusts the weight and sets his shoudlers. Steve is gone, but Bucky lives, and all Bucky knows how to be is Steve's protector. And if catching this shield is how he's supposed to continue guarding his North Star, well. He'll keep catching it, like he's always done, because nothing, not even death, could make Bucky stop shielding Steve Rogers.
