Let's put it this way: all the established companies who make and sell spearguns in Europe use the same barrell/shaft "ratio". That is: shaft=barrell (full band stretch) + 40 cm (so that a shaft for a 100cm eurogun is 140 cm, for a 110 gun it's 150, for a 75 it's 115 et cetera). They also give a tolerance of 10cm shorter than the standard if guns have a full length rail (100cm euro railgun: 130cm shaft)
The fact that ALL companies apply the same ratio, makes me think that this is the best, the most well balanced. You may think this is petty of me, trusting what big companies say, without a thought of my own. But they (the companies like Omer, Cressi, Beuchat, Sporasub, Picasso et cetera)invest lots of money and reputation, so I assume they know what they're doing when they sell guns kitted with a shaft length=barrell (full band stretch) + 40cm.
Redline: since you're an engineer who speaks italian, you will enjoy reading the article "balistica comparata delle armi subacquee, parte 1 e 2", that you'll find on the Maori website. Go to Maorisub - Entra, click on "articoli" on the menu on the left of the homepage and there you find it. Only italian version, unfortunately.