So I did some testing this weekend with my vulcan and the slugs and this is what I found. By no means am I a professional shooter so your results are going to be different than mine but never the less here they are. If you have any questions please ask!
That's the amount of drop from the 18.13 JSB pellets I normal shoot to the 27G slugs at 728FPS
Third group with the 27G turned up one full turn of my hammer spring so they were shooting at 728 FPS
Out of the tin JSB 18.13 pellets at 905 FPS. The gun is more accurate this ground isn't the best cause I'm not the best.
Second group with the 27G slugs st 728FPS
Second group with the 23G slugs at 799 FPS
Second group of the 21G slugs at 830 FPS
First and last group of the 33G slugs. Only those two shots hit the paper I'm guessing my gun just doesn't shoot fast enough for them and I'm not going to make it shoot fast enough for them.
First group of the 27G at 700FPS
First group of the 23G at 785 FPS
So these are the numbers. The top numbers are my gun as is how we started. The bottom numbers are after we turned the hammer spring adjuster in one full turn.