1075 caps are RWS and available here in the US.
You might want to try removing the nipples and putting each one into a hand drill motor, spin and clean up the top, out side edge of each, against a stone. Turn a bit then test cap fit with each. I think the bit of angular strike from your hammer face has raised a flat on the nipple edge that is keeping your caps from seating all the way down. It isn't going to hurt your used nipples any to try this. A little goes a long way here so test cap fit often.The hammer is making contact with the cap. Seems the impact is at the top of the cap.
Repeated impact causes the cap to have a bit of a mushroom only at the top.
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