Originally Posted by WildWayz
When I was at VG HQ on Saturday, Martin told me they no longer use Poly's - all the 017 inner can o-rings are now 017 Bunas.
I'm not 100% sure but I think they went back to the older can design since it was more trouble free. Basically built to different specs. The poly orings were used in the later 2011 guns to compensate for a minor size difference in the drive train. They've since reverted back to the older sizes and therefore the poly orings are no longer needed to prevent that issue.
That's how I understand it anyway. That was the only reason for the oring switchover. Different orings have different thicknesses/hardness/materials to compensate for the milling differences.
What does that mean to the gun owners? It means if your gun works fine with buna......use them. If your gun works fine with poly orings.....use them. If you have a previously owned gun and are having problems then I would try both and see which works best. If you are just plain unsure which you need, try them both and see what happens after a decent break in time.