nice having information on different forum just in case someday on of them goes away.
I havent tried the 850 I have shot the Walther 1250 which is more or less the same airgun (?) except for the 3500psi fill and several hundered dollars, maybe there are other differences and you can let us all know?
Other than ALL the white letter WARNINGS stamped all over it, the trigger after much work by a a fellow who knows airguns in general quite well ( which is where the "850 forum might help???) was awful, the safety had to be removed to get it under four pounds. Other than that it did group very well.
I use a Ninja paintball tank/reg on a QB .22 set from the factory at 850psi (not verified by myself just the claimed number) and with 15.9 JSB it shoots at 580fps. The Ninja is 2" dia. so "if" you have room on the 850 for a 2" tank the only other part you need is the AS to painball adaptor. A complete rebuild kit for the Ninja is $10.00 and includes a full set of adjustment washers. Naturally they do NOT recomend changing pressure.
Kepp us posted,
John