Not an expert but those o-ring are a pain in the neck to access (firing chamber o-rings) per a gun tech. For our application, urethane has very excellent abrasive and tear characteristics and good compression characteristics. Buna-N just has good characteristics for the above. Luxe is using a urethane70 (for this application) where other manufactureres specify urethane usually the durometer is usually 90 which is considered very hard material and 70 is hard.
Luxe makes a complete o-ring rebuild kit and is availiable from luxe vendors one that comes to mind is www.punisherpb.com
. They show a complete listing of the o-rings, sizes, material and durometer in the kit. I have bought o-rings from www.mcmastercarr.com
, they have them all.....in bulk.
Urethane is slighlty more expensive than Buna-n which is why it so widely used over urethane. Hope this helps. I have not substituted so I can not answer your last question, Buna-N may wear out faster??