For an FX slug liner, stick with the flat base slugs. The slugs with a diameter of .250” shoot best in the FX slug liners. For a 600mm liner I would stick to the lighter slugs.
On the other hand, my RAW likes the .249” slugs with a dish base in the 26gr-32gr. range but it also shoots .250” slugs well.
Others have recommended the dimple nose slugs, they haven’t worked well for me, but if they work for others, maybe you should give them a try. In my experience, there are many others that are much easier to tune and are more accurate.
There are also many lighter slugs on the way with both a dish base and flat base, they’ll be perfect for you. I’m not allowed to divulged their weights yet, Nick will let you know when it's time.
With all of that said, keep in my that some barrels shoot slugs better than others and some slugs shoot in a variety of guns better than others. I don’t like dimple nosed slugs but that doesn’t mean they won’t work for you, obviously some people like them. Some of the slugs I love and recommend may shoot terrible for you. Slugs rarely just work perfectly right off the bat. They really need to be tested and the gun tuned for them. With each slug I test, I shoot it to see what I think and then I start testing in the mid 850fps range and keep testing up to around 980fps or so to see what velocity the slug likes with my gun. I fine tune it from there. I have found that with the FX slug liners, the flat base slugs work best but I have also shot the 29gr Nielsen out of my Impact and it shot well. It’s both .249” and a dish base which contradicts what we’ve told you FX gun’s like. That’s why it’s so important to test as many as you can, you just never know what ones will shoot well until you try them out! I hope this helps a little. Good luck!
Stoti