FX Fx impact compact .25

I think the longer barrel would be beneficial to the harmonics of the gun. Shot cycle would be less harsh and would also use the heavy barrel liner as it would be stiffer right off the bat. I have only shot 27gr slugs from my 600mm M3 with good accuracy but at the cost of shot count and could definitely her the difference in the shot cycle compared to 18gr pellets.
 
I’m looking at shooting 40-45 grain slugs. My compact is around a 1.5moa with 26.54 hades. They just don’t have the hitting power I’m looking for. I’m going to probably go 700mm heavy barrel. and hopefully get the power I’m looking for at 125-150m.
I have no experience with slugs that heavy but would probably go with the heavy liner in your situation. I shoot light slugs out of my 25 cal 700 mm standard liner with success but haven't shot slugs that heavy.
 
I am shooting .25x700 variation of liners 100 meters BR rings. Tried for more then a year any slug over 10 K shots in between 37-41-44-46 gn and me back to 34 gn JSB MK1's. This up to 100 mM rings, again...
Also my view about replacing or upgrading the gun CF bottle - for the money I chose tehering with Huma externally adjustable Reg instead. In four years having an Impact version I never shoot it off hand so can't comment on those scenarios.