Slug recommendation

Pretty hard to beat the NSA offerings . Not sure if you can find a sample pack right now since he just moved the business. That’s where I would start. eBay usually has a huge selection but scattered from one brand to the other. I cast my own and have very rarely bought them. I do think that anyone serious about shooting solid projectiles should get the tooling for sizing. I use the NOE die and inserts as well as a Lee hand press. All in it cost me about 300$ but my results are well worth the expense. 
 
I detuned my RAW HM1000x LRT .22 to shoot 18gr JSB @ 950fps and NSA 17.5gr .2175 @ 935fps very accurately. You need to see what size slugs fit your leade correctly which should be snug not loose. If your RAW is factory tuned then 20-23gr slugs and get sample packs of different weights and diameters, which ones your barrel likes will be obvious right away. Good luck and have fun, great rifles!