if your out in field over winter when the light is failing then co2 is complete crap over here in the u.k
if you plan on doing any modding to the 850 convert to hpa it might seem more expensive than buying say an s410 new but you always have the option of back to co2 for plinking in the garden for practice.
roalds kit is very well thought out and will give you the option of a duel fuel rifle if such a thing exists
just my five pennies worth
simon
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