Both are spring operated mechanisms. They are an evolution to the gunmaker's highest art. I believe the only true self-opener is that of the Beesley action, like the Purdey. H and H claims they have one but it seems more assisted to me, having handled both. There are others as well. The drawback to a true self-opener is they are hard to close. I don't believe Win 21's are assisted.

Socialism is almost the worst.