Talon String Shot with Standar Tank for .177 and 25 barrel

Hello
I have an old Talon in .177 with the standar tank
The last week i bought a 24″ barrel in .25 and i put it
I begin to shot from down power to high. In power 2 it give 620 fps with JSB 25,4 grains,
In power 10, i obtain the best results, i can get almost 40 shots , the mwdia was 841 fts with the JSB King 25,4 grains
My results are:

Shot JSB King FPE
1 865 42,0
2 890 44,5
3 911 46,6
4 899 45,4
5 871 42,6
6 861 41,7
7 861 41,7
8 857 41,3
9 850 40,6
10 854 41,0
11 858 41,4
12 881 43,6
13 814 37,2
14 865 42,0
15 839 39,6
16 845 40,1
17 842 39,8
18 845 40,1
19 839 39,6
20 838 39,5
21 835 39,2
22 835 39,2
23 835 39,2
24 837 39,4
25 832 38,9
26 825 38,2
27 825 38,2
28 827 38,4
29 826 38,3
30 838 39,5
31 826 38,3
32 838 39,5
33 818 37,6
34 819 37,7
35 814 37,2
36 813 37,1
37 810 36,9
38 814 37,2
39 810 36,9
40 808 36,7

Velocity graph

On the other hand i test JSB King, BEnjamin and Skenco 31 grains
But i can´t get groups with Benjamin pellets (it spread like a shotgun, almost 20″ at 40 yards)

With JSB at 40 yards half inch groups are doable

and other group show a vertical Shift (may be influenced by the speed)

And the skenco, demostrated are good pellets in this barrel

But now i Have many questions
I think that a 500cc air tank can get more shots at 41 fpe… It is possible?

1. What i can do, I´m looking air efficency…
2. If I use the standar Tank for .22 (not for .177) i can get more shoots? or i can get more power?
3. If i put a hammer with more weight, or a heavier Spring, can i get more shots in the lower power but gettin 41 fpe?

thanks