My B9 shoots RWS Meisters (7.0gr) at 490 quite consistently. I've put about 1000 rounds through it now, and though I 'tighten' the locking ring every once and a while it's never really been necessary as it has never been loose.
Perhaps yours is faultly.
I have two friends who have the same gun and are happy with them...I'd hate to see a good gun get a bad rap because of what could perhaps be one faulty unit that gets posted on a widely read forum.
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