speargun bands

I bought a speargun on craigslist (27" nice and short for rockfish while looking for abs) and made new bands for it. It came out great. My friend has an old speargun but no bands. I want to make bands for his but he doesn't have the old ones for me to measure. Is there a rule for band length for a certain length gun? This gun has a 20" barrel and is 31" overall.

http://www.spearitco.com/index.php?main_page=page&id=10&zenid=bf82a35a21570d4fcaa44ace78793192

-measure distance from muzzle to loading tabs (your band pull length)
-divide by 2
-add 2 inches to compensate for lost band length due to constrictor knots.

this is based off of 4 inch wishbones.

Hope this helps

Thanks

Excellent link. Thanks.