Lets see how your compression area looks like when nothing in:

Now when the piston (with seal) inserted there look to be big difference. Yes, almost 24mm (wastage) in length.

What I have done next. I made the piston small by made dovetail front part of it like this:
This thread has been edited to conform with Indian For Guns policy as stated in the Announcements forum
By which I gained fair amount of volume like this:

Now this is how it looks the final product looks like:
This thread has been edited to conform with Indian For Guns policy as stated in the Announcements forum
Further more: Most of DYI guys know the fact that IHP having huge preload which doesn't enhance power. You can reduce preload by inserting this tube behind the spring guide:

Any after that you need spring: Here is the specs:
Wire Dia: 3mm
ID: 12.7mm
Total coil: 39
Hope this help. Good luck!