it shot sluggishly because 1, its still being broken in. eventually, if you have not yet seen any black sludge looking gunk on your bolt, you will a few times until it is fully broken in. And 2, you were correct about the red silicone rings swelling. Buna 12's work, as you discovered, but the best fitting o-ring is going to be some metric variant that is an off the wall size. Several members are working on that now, and you can see that discussion a page or two back in this thread.
your doing everything right, no worries just keep on shooting with it.