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When I was researching the s510 .25 I came across some some threads on the Regal .25 and it performed basically the same. Pretty sure you can expect around 20 shots per fill at 38-39fpe. Looks like Daystate did the same thing that Air Arms did with their .25. Slapped a .25 barrel on their .22 rifle and called it a day lol. If you don't mind the lower fpe and shot count I'm sure it's a great gun and as accurate as the rest. My s510 was excellent and very accurate, I just got tired of filling it.
Edit: I should add that you can probably get more than 20 shots but with a higher es. So maybe 30 shots if you're shooting within 30 yards and 20 shots if you plan to shoot longer distances?
Straight shooters has some pretty good info on the gun you can check out also.
I had one that I traded for a Wolverine .177 for field target use, figured an update to the new chassis was worth it. the Huntsman would never hold air over night, even after a warranty "rebuild".
I have one on order. My airgun gunsmith will up the power to about 50ft/lbs for about 40$ so I believe it can't be too much of a hassle if you need more power. It is true though that shot count drops drastically to about 10 or so with low enough es. I use it for hunting so shot count isn't really a problem for me.