The "proper" name for a air rifle gas ram or nitro piston etc, is Ta da - gas spring. They perform exactly the same job as a steel spring. The main advantage a gas spring has is that it does not vibrate so can be quieter. The recoil they produce is more or less same as a steel spring. Hence I would not put NV on a gas springer or a steel springer. I think the Gamo air rifles are overpriced for the accuracy and quality that you get.
Most spring rifles excepting youth and target models have the power to kill a rabbit at 50M if you shoot it in the head, hence accuracy should be your first criteria. Possums are a bit tougher, and a bit of extra power would not go a miss.
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