I'm not familiar with the differences in materials. are they the same product?
What is HNBR? what is buna? do these rings work better or worse? what is harder wearing?
as an aside, I've used SCUBA tank rings on my air tanks for years. they outlast your standard tank ring 100x (this was in my spider days before on/offs), but that simple change in o ring technology was a second coming for me. that being said, if there is another ring type, I'm all interested to learn about it's potential.
I have a G6R and I love it. I'd love it more, but the barrel is too small.