Personally I currently use the 3 grove 3/16" Shurz-A-Peep and keep it inline with a ultra-nok II, Although aloop would work just as well. The reason I don't like the peep's with rubber tubing is they seem to fail when you need them most. I've tried three different types in the past and all with the same result, They break! It may be because of the amount of shooting, the major temperature changes we get, or getting caught on a bush. No matter, the way I see it the less parts to a peep the less chance of something going wrong.