I help coach the MIT collegiate pistol team, but end up spending half my time doing gunsmithing. We have a whole raft of match CO2 pistols that need occasional O-ring replacements, and I'd like to make a custom bench block to hold some assemblies while I work on them. I have a couple of general purpose commercial blocks, and I _think_ they are made out of Nylon.
I'd like something easy to machine, but tough enough to put up with some pounding. A couple operations require pressing very large snap rings on/off or pounding out cross pins with the piece supported mostly on a coarse sharp threaded section. I don't want the block getting chewed up too quickly from those operations. I have thick blocks of PVC, but I think that is too soft.
Likely suspects would be Nylon (several flavors), Delrin, or UHMW polyethylene. Extra points for something I can get easily from McMaster Carr in ~ 1.5" x 3" cross-section. They indicate that polyethylene has better impact strength than Nylon or Delrin, but I'm not sure that is the best metric.