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Fx Mavrick. 25cal 700mm cant put two shots within 6 inches of each other

Did you put the liner in the right way, as I believe they are directional.

There is a band about a 1/4" from the end on the breech end. If you put it in backwards, you get the choke first then it opens up causing " two shots within 6 inches of each other ".
 
Anything under 34-36gr is too light for the heavy barrel, the twist rate is too high. You haven't mentioned the speeds, you are shooting either. And the barrel in backwards of course.

I have shot lighter slugs (26 & 29) in my heavy liners with great results, for slug it’s better to have too much twist as supposed to too little. Even 34 grain heavy and hades pellets shot awesome with the heavy liner.