There are a few reasons that may not be apparent on the surface, I will explain and hopefully that will shed some light on why we have a different bottle here in the states.
1) What may not be clear to everyone is that the 400cc aluminum bottle used in many other parts of the world on the Avenge-X only fills to 250 BAR. Because of this, when you compare the 250 BAR fill bottle to the 300 BAR fill 210cc air tube, you only see a 5-10% shot count increase if you're comparing the same tune. Not a worthwhile increase in my opinion. Compare that with the 300 BAR bottle offered here in the US, and you get a much better ~40% higher shot count over the tube.
2) There are not very many 300 BAR bottle options in the diameter that it used on the Avenge-X (in fact, I am only aware of 2 others). Because of the modular design, and the fact that the drop block on the rifle accommodates both the tube and bottle without additional parts or modification, going to a larger diameter bottle was not in the cards.
3) They call it out in the video, but the rifle with the 250 BAR alu. bottle fills via a fill probe. You will notice a QD on the neck of the bottle, but the position of that QD is not easily controlled (meaning it doesn't always end up where you want it and can access it) and on some models of the rifle, it would be covered by the stock itself. I know fill probes annoy (that may be putting it mildly) most of us, and that was a concession we were not willing to make with the Avenge-X for the US market.
FWIW, based on feedback from a number of places (AGN included) we have a cap/cover in the works for the bottle that will alter the appearance to make it look more traditional.
Hopefully that adds some clarity on the bottle. Ultimately, we wanted parity (both in features and functionality) between the tube and bottle versions in a way that no one else had done before, and I think we have achieved that.
www.pellet-guns.com 
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I really like the gun so I hope I can keep it going..
