Re: What if my necks are tight enough to hold a bullet firmly AFTER firing..
Remember, when Virgil was shooting in the warehouse, he was using smaller type cases that do not hold as much powder and do not produce as much "ash" as the bigger guns. With the larger guns that produce healthy "ash" deposits, you might notice the necks getting every so slightly tighter the more you shoot (that is until you clean) if you are not sizing your brass, that could lead to neck tension that is not equal across the shots you are attempting to group.
Non-uniform annealing will cause neck tension variances as well. That is why some guys at BR matches shoot their best groups on friday when they are forming brass for the weekend........ Specifically, the virgin brass is very uniform in neck tension when turned to the same thickness.