In my RAW HMx LRT .22 it likes NSA 17.5gr .2175 & 20.2gr .217. It seems the slightly longer 20.2 do not need to fit as tight when loading as the shorter 17.5 .2175 as the 17.5gr .217 do not group as well as the 17.5gr .2175. The longer slugs stabilize quicker which are usually the heavier ones.
I don't shoot slugs, nor do I currently own a poly barrel, so I can't really contribute, but I have a question. Given the poly barrel profile, and the longer bearing surface of slugs compared to pellets, it seems to me that lead fouling could be a problem. What is the general experience?