Need Help Texan .308 Max Shot Count Light slug?

So I just bought a texan .308. I wanted to get the most fun shots out of it even thou it is not a gun for paper punching. I am curious if anybody has ideas to get the most accurate shots out of this gun.

I am not looking for power per say as anything this gun shoots will be more than the average .25 ca.

As of now I am shooting JSB 50 grain bullets and 44 grain round balls.
I been able to get a string of shots with 50 grains JSB starting at 2100 PSI down to 1600 PSI of about 20 shots. The FPE went from about 77 FPE to 100 FPE. Do you folks have any suggestions of how to get a tighter FPS within a 10 to 20 shot group? As you can see from the numbers it goes up and then comes down over 20 shots but the spread is about 100 FPS. This seems a bit radical.

That said I am curious i there is a way to put a regulator on the gun to maintain shot to shot performance?

I wan to try to create a condor on super steroids with the most accurate shots at 100 to 120 FPE. Any tips would be great.

anyway here goes the top 20 shots going from about 2000 PSI. I turned up the power to about mark 2 out of the 5. or basically the first mark line.

2000 psi
1. 818
2. 803
3. 820
4. 785
5. error
6. 811
7. 833
8. 833
1850 PSI
9. 855
10. 857
11 861
12. 895
PSI about 1750 to 1700
13. 855
14. 882
15 885
16. Error
17. Error
18. Error
19 900
1500 PSI
20. 999

Any tips on tuning.