.50ths -.55ths ? does the ths mean thousants of an inch so it would be .050"-.055" OR is that .50"-.55" ?
everytime I see the situation on the gun forums,it's either corrected by the poster by increasing the crimp or decreasing the diameter of the stem that goes in the case with the 'bell mouther'. even without a crimp,there should be force required to push the bullet into the case,'neck tension'.
Maybe an inconsistent crimp due to inconsistent brass length. a shorter case,even by a few thousants,would have a weaker crimp because,then, the case isn't going up into the crimping die as far as the others did. All my brass gets length trimmed only because I want a 'consistent pressure' crimp.
On my 'nuclear' 357 loads,stricly a display photographic load,way past the max recommended with 2400,I would typically just load one because that case would have to be 'hammered out' due to extreme pressure. whenever I did load more than one,I never had 'bullet jump'. for the 44 nuclear loads with 2400 and 4227,I also never had bullet jump.
How many rounds in the cylinder were fired before 'cylinder lockup' ? was it after the 1st one or after ? ? If it took more than one,you're gonna have to load two,fire one and measure the remaining one to see if it changed in length.
Are you working with a progressive press or a single stage press. Because a progressive does multiple processes 'per stroke',it's difficult to sense/feel the crimping pressure as with a single stage.