Forget the 6.5mm shafts. A decent fish will bend them like a pretzel.
I assume you will be diving in 3-4m viz?
Make your gun ~75cm from muzzle to triggermech. Site the handle towards the middle of the gun. This will make it nice and easy to swing around. Use a nice solid shaft, 7.5mm or 8mm, 1m long, plenty of punch that way. Twin 16mm rubbers (each rubber 50cm total length, so 25cm a side) for plenty of shaft speed. If you are making your own gun, then you will probably use a bulk rubber muzzle (see Riffe guns for what I mean), which is way cheaper than screw-in rubbers which require you to buy a muzzle.
Rubber length is calculated using the 3:1 stretch "rule". So for a 75cm gun, a rubber that is 25cm will stretch to 75cm, or 3:1
And heres a gun building tutorial by the Sultan himself:
Build Your First Speargun - A Beginners Primer by Sven Anderson on DeeperBlue.net - Fanatical About FreeDiving, Scuba Diving, Spearfishing & Technical Diving