Common Gull dropping sea shells on the sea shore

Today at Crymlyn Burrows I watched in fascination as a Common Gulls used a surprising method to feed. I was at the western most part of the beach near to the docks. The beach here is firm underfoot with a strong shingle element mixed in with the sand. It was about 3 hours after high tide with good numbers of birds feeding at the waters edge. Oystercatcher numbered about 200 with a few Curlew also. The remainder was made up of gulls. 60+ Black-headed Gulls, 30+ Common Gulls, 30+ Herring Gulls and also some Greater Black-backed Gulls. I noticed that a few Common Gull were flying up into the air and coming back down and then up again. On closer inspection I could see that they were taking up shells and dropping them onto the beach to break them open. If the shell did not break open then they would swoop down and pick up the shell and drop it again. It was difficult to see exactly what the shell was but they appeared to be the size of a large acorn and pale in colour. It looked like some kind Sea Snail? The Common Gulls were the only gulls to behave in this manner and although immature birds where also present, only adults were seen to perform this trick. Presumably Common Gulls do this in other locations locally also, if so and you know what the shell is I would love to know?