Since most of the o-rings are static (non-moving) they can actually be off a size or two. Though in this particular case it's better to be a size under than a size over. Over sized o-rings find their way in the assorted holes you'll have to push them past.
And nope, no detent. It is a breech drop though (if memory serves me correctly), so the ball has to go up a little lip to get into the barrel. Once in the barrel, well, all we got is friction.
Nunquam emere quid potes facere