Umarex Umarex zelos bottle conversion

Just ordered one of these, my question is. What's your shot count with the .25 zelos reg set at 1,500 to 2,000 with the original tank, and whats the shot count now with what size bottle. I'm debating on going with a 500cc bottle like what most people do or maybe smaller because I would like to hunt with the gun also and would prefer to keep the gun lighter and smaller. I go quite the way when hunting so the estimated 20 shots with the stock tank feels not enough to be away from any pump all day or couple days. Haven't been able to shoot my zelos in stock form yet but looking for info.

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