Hiya
Carbon is prefered in lengths exceeding 110cm's is euro type guns, due to carbon being stiffer than alluminuim, therefor flexing less. How-ever, depending on the application, a alluminuim gun in 120cm length will work just as well. Personal opinion is that the alluminuim gun will be a little heavier, thus being able to absorb more recoil. Also, the RA guns have a extruded rail, making the barrel stronger and stiffer than a normal euro type gun.
Set-ups. For smaller, fast moving or flighty fish, a 6.6mm RA spear and a single 16mm or 18mm band will work well. For meduim to larger sized reef fish and smaller pelagics, a 7mm and a single 18mm or 20mm band will work well. For a little more power when hunting very large quary, a 7.5mm RA spear with 2x16mm's will be perfect. With even this set-up, you'd still be ok with a 120cm Alluminuim gun.
Full range would pretty much be the same as on any other 120cm railgun, which is about 6.5m from your hand.
Railguns have always be reknown for their accuracy. Depending on your set-up, a single 16mm and a 6.6mm spear will be more accurate than a 7.5mm spear with 2x16mm's.
Regards
miles