I love HNBR orings, but everywhere I order them, including OringaUSA. They do not run larger. If anything they are more consistent in size which is both good and bad. I like a little fluctuation in size, as I can go up or down depending on fit. Say, inside the can or the bolt sail.
They are better than Buna for durability, but buna gives you a larger range. (why?: I dunno - as I would think it would be the same cast or mold)
Urethanes are ok, but as you said they degrade faster than buna, and HNBR. (from my experiences) either getting old and yellowing, or becoming stretched out and loose like you said.
So no worries get some HNBR's and rock them out!
My Current Markers:
(Exclusively Using Monkey Poo Lube
/ Gun Sav Lube
MacDev Clone GT
Black/Green = Ninja Air 550psi 70/4500 = Seed Rotor
Black/Green (Pooty51 Polished NDZ Can/BubbleGum Flat Face Bolt) = Ninja Air 550psi 70/4500 = Seed Rotor
Black/Green = Ninja 550psi Reg'd Crossfire 70/4500 = Seed Rotor
DigiCam Midnight (Virtue OLED) = Ninja Air 68/4500 = Seed Rotor
Midnight = Ninja Air 68/4500 = Seed Rotor
Champagne (Pooty51 Polished Bolt / Virtue OLED) = Ninja Reg'd CrossFire 70/4500 = Seed Rotor
. . BattleField Orlando