Generally, a ring is safer than a barbell, as the ends of the ring shelter the ball on two sides and make it very unlikely for it to come unscrewed...
If using a screwed ball piece of jewelry... make sure that the threading on the ball is VERY FINE... the finer the pitch of the threads, the tighter you will be able to screw the ball by hand, and the less likely it is to come unscrewed.
Safer still, Is a captive bead or segment ring... these hold the ball or segment with spring tension, and require a pair of plier-like ring spreaders to even insert the ball or segment.- because there is no force in fucking that could possibly act to Spread the ring apart, there is zero danger of the ball or segment coming loose, as long as the ring has proper tension. ( it is possible to bend the ring in removing or inserting the ball/segment... such that it does not hold the ball or segment tightly ... but you should be able to immediately notice this when you first put it in. )
I fucked happily with a captive bead ring in place for 20 years without a single mishap.
The bead was held so perfectly, that it actually was free to spin in place...but could not come loose due to the ring ends setting into recesses on the ball.