So what are you trying to achieve with a 177? Are you trying to have your slugs high power? Like 16gr and up? Or something along tge lines of 13-15gr?
The FX liners in 177 work phenomenally.As long as you don't have your expectations set too high.
I've taken iguanas as far as 144yds with the .177 500mm barrel liner. As long as you get the 1:18 liner which everyone who has them sells them you will be able to shoot 14gr skugs and under phenominally. 15gr and higher you will need to set up a hot rod tune, because for some reason you need an inefficient tune for then to group.
I've consistently hit golf balls with the 13.43gr JSB KO at 120 and 130yd. A local buddy of mine has the rifle, but
@Scotay42 has the barrel, not sure if he's used it yet but with the 2 .177 barrels I have had 500mm and 600mm you can shoot slugs just fine. Now if you want to shoot 16gr slugs and higher you'll need the 1:16 twist heavy barrel for the impact.
I've spoken with
@mercado and he's had the U1 .177 for years and he sling 16gr slugs at 970fps accurately and without hiccups.