yeh, it is a bit different. delrin will swell slightly, if it gets oil on it.UHMW and Delrin are a couple different things. UHMW is polyethylene and is quite a bit slicker than Delrin. I would think Delrin would hold up better to the abuse of rocks though since it has a better hardness than UHMW
i believe UHMV is pretty much NYLON. worked at a vinyl window plant, we used it alot for the tables, since its so slick, and durable.