Primer not seated correctly (too high so when the bolt locks up, instead of a light normal dimple from the FP at lock up, it ignites the primer instead), disco failing to retain the hammer until trigger release/adjustable trigger not set correctly, or even a blow primer piece under the trigger that does not allow the trigger to be pulled fully to the rear can cause a run away.

As for the case not sized correctly, not going to cause a run away.

Granted that I have had to face surface a few full sizing dies over the years to take off around .010", most of the time with sizing dies and a single stage press, the problem is that the full sizer die was  not adjusted correctly  to kiss the top of shell holder under actual ramming conditions instead.


The new to reloading lad's will do a quick adjustment of the sizer in the press against the shell hold with the handle down for a light kiss that way, then when the go to actually full size a case with tension on the handle/linkage, end up with a huge gap between the bottom of the sizer and shell holder with the handle down and the case not correctly full sized instead.  

Also to point out here, even with a correctly full sized case, you can still screw the pooch by over pressure crimping the bullets as well.  Here with too much bullet crimp pressure used, it ends up buckling the shoulder of the case instead, Hence the problem most of the time here, cases are not unformed trimmed to begin with, and although the needed amount of light pressure bullet crimp may have been set correctly to begin with, a longer case was mixed into the mix instead. So on that note, if you are bullet crimping, and not unify trimming the cases each reloading, then get a Lee FCD to bullet crimper instead.


Seating primers, like to think of the little extra pressure needed to make sure that they seat all the way in (and not flush with or above the rims), is also giving the primer cups a light squashing to seal tight against the primer case channels as well.