The lifter spring in question, has 2 functions. It holds the lifter down to allow shells to feed on top of the lifter. (Also to allow loading the magazine) The hidden function, is the articulating pawl that engages the slot in the bottom of the bolt that actually raises the lifter. The function of the pawl is not dependant on the weight of the lifter. If the pawl had enough spring to function with a factory lifter, it will function with a modified lifter. Any time you have a part of a machine modified without verifying fit and function, you are asking for problems. Most all modified lifter issues are directly related to the modified shape placing the shell in the wrong location/angle.
The gas system is a short throw, because the gas system does not cycle the action. The gas system just unlocks the bolt at the correct time, then the remaining pressure in the barrel continues to cycle the bolt. The heavy piston delays the unlocking of the bolt a little longer so the pressure can drop down from the heavier load. The lighter piston does the opposite. It unlocks the bolt a little earlier to give it a little extra pressure to cycle the bolt. In theory, with excellent engineering, the force from the gas system on the bolt should be the same with both pistons (when used with the proper load).
If anything, it gives me more appreciation for competent gunsmiths, that actually fit parts, rather than hack a single part up, then ship it off hoping it will work. I'm not certain what you mean by shooting a self loading shotgun upside down, but the shotgun would not know the difference. The only difference would be the effect of gravity on the shell its self. The fact that the shell if forced out by a spring, then held in the lifter by spring tension, then forced to the top of the action by the bolt acting on the lifter, and the hump on the lifter holding it to wards the inside of the chamber, and the bolt forcing the shell to the chamber..... what could possibly go wrong?
