You guys should try the Durango blue S30 orings. Direct fit drop in on the D413 shock and just as smooth as the traxxas xring and no air leaks or funky shimming needed.
http://www3.towerhobbies.com/cgi-bin...&I=LXDXAE&P=ML
I've tried them both and the Durango orings are the better solution. Just as smooth, alot less hassel.