I am building new multiply or inverted roller what ever the name ,not interested in that kind of speargun but it was a challenge with my buddy ,I almost finish and I just need 2 small info , as per the sketch where I should put the anchor for the rope which come after the pulley ? before or after the roller pocket ? second how to calculate the rubber length ? its 7mm spear 120 cm length ,2 pairs of 14 small ID rubber .
Id run rubbers as short as possible meaning maximum band stretch. 400% (4x as long as unstretched band)
pullys will cut the band power to the shaft in half so load up at least 3 pairs of bands.
make sure your anchor points, rollers (sheaves and axle), as well as your trigger mechanism can handle this load.
Definitely play around with the number of bands and thicknesses of bands with this one, there is even less information on these types of guns, band setup.
still need more information how to get it accurate for that 7mm 120cm spear
figure out the maximum band stretch you can have. Then divide that by 4, this will be your band length. These guns are interesting in theory. Although the pulley system cuts the amount of load on the shaft in half, the same amount of work is being exerted on the shaft in motion. This results in a smoother shot utilizing the full length of the barrel, as well as the full stretch of the bands, without any noticable recoil. This means you can use large diameter shafts on a relatively low profile speargun, all you need to do is add more bands.
As far as accuracy goes.... youll have to play around with the gun, no prototype gun is accurate without field testing, and tinkering.