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I went to lebarons at woodbine (5 min from work) last week looking for some regular BC caps (which they didn't have), and the guy showed me some of these. I was unfamiliar with them so I didn't buy anything that day, and got some regular caps from cabelas online. I could pick some up for you and meet you at the range in a few weeks if you'd like.
You may be thinking of a different kind, these are made of rubber not plastic. They're superflexible, I can't see them breaking until the rubber gets real old.
I can bend them and twist them, they just go back to their original shape. They don't stay closed as well as the plastic ones, but they are a heck of a lot more durable.
Well, it seems that these don't work very well on the eye side of the Vortex PST 1-4x24 scopes because of the rubber ring around them. Its also soft and flexible, so it doesn't support the cap.