If your just starting out then forget about double theraband, just use single theraband cut at 25mm wide and use 9.5 mm steel ball bearings as ammo or m.8 hexnuts, I never use flats anymore as they last no time at all, I use looped 1745's with m.8 hexnuts for all my hunting, tubes also last about 10 times longer than flats as well, I've got a nice catty for sale that comes with looped tubes if your interested? Cheers