The inner steel barrel insert contracts and expands at a different rate than the outer aluminum shroud, leaving a pocket or tight spot for lead to build up in.
Rugers' fix for it without admitting a problem was to come out with new pistols with vent holes on the shroud, and give them fancy colors to market them as something new, so no one would ask questions why they discontinued the fluted ones.
They leaded up suppressed and unsuppressed.
I have had a few. Rugers final letter to me said not to use lead ammo
and not to use it unsuppressed. .
ETA: I still have a cobalt and want a blue one so don't think I just try to shit on these guns. I really think they are perfect when the work as intended.
ETA2: the cobalt does come with a rail, just not pictured.