Order of the testing may also have a bearing on the results as the rubber being stretched may not completely recover after each shot as the test series progresses. So shots made at the start may be different from those made at the end due to this rubber elongation effect. To determine if that is happening a reference set-up needs to be tested throughout a test series, intermingling it with the other tests.
I was also worried about the possibility the spear gun would lose some air. But the test with small stainless steel slider was just after the free shaft test, and the difference in results was huge. I just removed the piece of line for the"free shaft". There are some more factors that can influence the measurement, like the sliding knot tension, and the waves in sea during the test, temperature of the water (influence on rubber and the air in the spear gun), moving of the water because of the wind...
Rubber (surgical tubing) was under tension very short time and could recover between two tests. I did one test ( one setup) usually in the morning and the other in the evening, so measurement last for a few days. I suppose the results are quite reliable.