Where did you hear that? It seems to me that the only thing that matters is the relative speed of the two components. Was the ring much lower mass than the core?
Edit: it was more. Nonetheless, accelerating the ring had advantages, according to WP! I didn’t know that.
It's not the mass per se, but the need for the thing to be contained at the target end. If the ring is already at the target end, you have a relatively large mass of enriched uranium surrounded by a neutron reflector. That's not going to make things go bang, but it is enough to be rather exciting. By sitting the core cylinder in the middle of the containment vessel with a gap around it and filling that gap with the ring on firing, nothing goes critical until it's supposed to go supercritical. (The ring is carried on a sort of sabot.)
Edit: it was more. Nonetheless, accelerating the ring had advantages, according to WP! I didn’t know that.