from various models, experiments and other and sources, headspace at the point of bounce can be anwhere from 0.5mm for the most extreme, short stroke beasts to around 10mm. for a well set up gun (for 12 FP), I'd go with around 1.5-2mm.

The amount of bounce can be anything from 10 to 20+mm

TopDog on here (who we had the pleasure of meeting at the bash) has measured piston displacement directly using a potentiometer arrangement, to get real time piston position graphs. Very interesting