Tuning IMPACT .25 Tune

I'm shooting the Varmint knocker 34gr at 1025fps and get great results out to 100 yards, farthest I've shot at the range with this setup. I have the 600mm with the heavy liner and am running the DonnyFL Ronin with the extension to really keep noise down.
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I forgot to add that the twist rate will play a part as the heavy is a 1:18 vs the plain superior at 1:24. Hope this helps out.
 
So I have a 600 mm barrel with the superior liner not heavy I'm sure in the 34 g NSA Slugs trying to get them out to at least 200 yards but I seem to be losing accuracy at over 80 which doesn't make sense 

Was trying to see maybe with some caliber measurements I can tell my air and reg in if anyone had the same set up




From what I have been reading and saving notes on, having the same setup as you with the 600mm Superior, you should be launching the NSA 33.5gr closer to 980 to 1,035 FPS. I tried these before adding mods like the PP, slug kit and modified dual ports and was not impressed compared to the AVS slugs in same weight class. But, they were not awful either being just over MOA when pushing them on 150 bar with no mods at 920 FPS. I may revisit them again now that I can easily reach the higher speeds. I have mainly been focused on the .30 cal 700mm barrels this year.