I'm looking for a Stucky fic that I believe is part of a series or in the latter half of a long story (I feel like Clint/Coulson is the other main pairing?), where Bucky is Steve's Omega/Sub. When they have to bring the Winter Soldier in Bucky wants to kneel so badly that he punches his own leg with the metal arm to force himself to kneel. At the end of the fic Steve goes out to buy a collar for Bucky - it's brown leather (and he remembers that, when they were younger he had to use a belt because they were poor) and has a detachable plate (like Clint's) so he can fight in it.
I've tried searching AO3 using various tags with no avail, so any help is greatly appreciated! Thanks in advance!