there are probrably a dozen or more manufacturers of those rings so know which are better than the next is a real crap shoot. get several sets, and even the bolt upgrade kit. good spare parts to have on hand
Check out this video: BCG Assembly
About half way through the video is the info on changing out the gas rings.
All of these videos on Brownelle's website are GREAT info.
Brownell's is also a GREAT place to get the individual parts. Sell to the public, no min. orders, good prices, good people.