I would tilt the bottle, depending on what regulator you're using (you need a low press reg to lower pressure in bottle 50ish bar to system pressure 10bar) A horizontal bottle may let liquid CO2 through the regulator - freezing it - which could limit it's regulating ability. Also seals on valvetry and attached cylinder etc will not like liquid CO2. If you dont have space to tillt the bottle you can use an anti-siphon tube inside your bottle.