I'm unaware of any 12-15 grain slugs for the .177, where can you pick up such animals?
As for my experience with the Avenger and slugs, I have the .22 and it just didn't have enough oomph for even 22 grain slugs. Best I could get was about 850fps where my goal was 900 fps with the 26 grain slugs. One of these days I'll tear down the entire gun and see if there's a blockage, but for now, just didn't have the power. I'll convert it to a standard pellet shooter and keep looking for my slug gun.