Its a spring and detent - Dark Storm Industries Bolt Catch Spring / Detent Kit

When your favorite bolt catch doesn’t come with the spring and detent. Grab this or maybe 3, for some reason company’s don’t throw these in. And don’t get me started on the bolt catch roll pin. It’s like a gas block that doesn’t come with a gas tube retention pin. A lot of 4 letters words that come spilling out. All in all the fit of spring and detent will work perfect for whatever bolt catch you grab. Grab 4 or 5 you’ll eventually need them.