FX IMPACT M3 SLUG LENGTH QUESTION?

[Rustoleum]

Just ordered my first FX Impact M3 in 35 cal. from Utah Airguns.
I did order the 800mm STX Slug Tensioned Barrel Kit
Now I just need to wait about 4 weeks for it to land in the US.

I am looking to cast my own Slugs.
What is the Maximum slug length that will still function without issues?
Anyone have any Favorites for this setup?

Thanks.