It's Worth the Work
Unless you know someone who is shooting the same gun, you should invest in buying several different boxes of slugs. I know that is expensive but, it will save you money and diappointment in the future.
Remington Premier Copper Solids have been great for me. However, I tried both 2 3/4 & 3 inch versions, Lightfields and a couple of other brands at the time. Any one with a new gun should go through this exercise as, even "identical" guns may have different preferences in ammo.
Take your shoulder pad or get one before doing this. Make sure you have a solid rest in order to take as much of the human element as possible out of this. A friend and a decent spotting scope help at the range. Let the barrel get cold between brands. Look for the best group. A tight group can be moved. Consistancy is the key.
Have fun!