Wow! So you're building a 6kg robot and get yourself a 1.7kg tank! As I previously explained, you don't need a tank that big/heavy, a 1/2 kg of liq co2 will last a lifetime for the size of ram you plan to use. Also as I have already told you, the thread on a soda bottle is unusable, you need an adaptor like this:
http://www.ebay.com/itm/Sodastream-B...-/161034825292
which will cost you more...in the end, it would have been cheaper, better, lighter, smaller to get a 9oz paintball tank!