I'm not sure either of them is the right approach to be honest, what are your actual input conditions (In terms of force, pressure etc)?
Are you using pressure inside the casing to push the casing out and into the barrel?
Results could also depend on whether or not the barrel is 'rifled' (Spiral bored).
This is more common on long range firearms obviously but can be found on pistols also due to it's profound improvement in accuracy.
Just food for thought.
not really knowing the subtleties of this it would seem to me that the second approach with the gap would allow the casing to see a hoop stress that would result in a lower load on the barrel locally. You could a do a hand calc and figure out the hoop loads and see if the hoop load ratios line up. Seems unlikely given the wall thicknesses of the parts involved and their stiffnesses. I assume you are reporting the peak stress which might not be the metric of choice to see if the effect is real......