On the question of case filing and unused space. There have been many discussions and opinions about filling unused space in certain cases, the .38 special and .357 case, straight wall cases as such. Vihtavuori Manual warns that "when using a large amount of free space the shooting characteristics can vary widely depending on the location of the powder.In some cases can actually double the pressure. Can effect velocity changes between first and following shots. Recoil can reposition the powder in the case having the same effect. Powder position can be altered just by position of the muzzle direction and movement of the gun. The shot to shot deviation in velocity and pressure are normally increased when using a load that leaves the cases half empty, there fore not recommended for target loads"
The risk of under load detonation becomes a risk, especially when using loads with any of the smokeless powders and can generate a burn of a shock wave to detonate instead of a normal burn of the powder that could cause damage to gun or injury to shooter. T
This theory has been around for some time and there are many arguments pro and con about this. Some call it flash over. If it is true it can definitely effect velocity/pressure and in turn accuracy.