Not sure what's happening here..slugs??

I had a friend give me some slugs... said he was sick of trying to tune/find the "one" 

Today was a slightly breezy day... no big deal.. I decided to try these slugs that he gave me....17.5gr .2165 nielsen slugs in my fx dreamline compact... 22..380mm barrel. I have it tuned to shoot jsb 18gr 853fps.(groups are great) Reg set at 125bar.. hammer spring on 3. When I go up from there the fps doesn't change....it just waist air.. So with the same tune... shooting the 17.5gr slugs... it starts to group ok..wonders some...it wants to group... ok??? I set up my chrony it shooting 832fps with slugs.. just as I was thinking this little compact just wont shoot slugs..it wasn't built to do so anyway just give up and move on... I crank up the hammer spring to max and shoot... the groups tighten right up... fps stayed exactly the same!! The groups still isnt as good as the pellets... close though... I'm not sure what to think here.... what waist air shooting pellets and cause my pellet groups to open up...make the slug groups tighten up?? What does this mean... it may like the 17.5gr .217 better than the .2165... does a half a thou make that big of a difference
 
Usually accuracy from slugs is due to spin stabilization plus enough speed to keep it spinning, so the better the slug contacts the lands usually the better the accuracy. 1/2 a thou can make a difference, so, you might want to try .217 and you'll quickly find out! I shoot the NSA 17.5gr .2175 out of my RAW, the .217's where nowhere near as accurate as the .2175's.
 
Find out what the groove diameter is by pushing a pellet thru the barrel and measuring it's od. Example, my.25 Sumatra's barrel slugged out at.2515. I size my slugs to.252 and they shoot great. This is the procedure that Roach Creek told me about how to get slugs to shoot correctly otherwise it's just a guessing game.

I took your advice and done just that... it slugged .2165. Which are when same dia slugs I have. So I got with my buddy who's over chasing the slug dragon to see if he had any 17.5gr 217 slugs. He did and I bought the box (new unopened) came home shot a few and started tuning. Next thing you know my compact is shooting those slugs at 848fps very consistent and most of all very accurately. Holding groups just as well as the pellets did. At 50 yards. I haven't grouped them any ferther yet.. I have noticed that the slugs do fly more flat..after test and tune I went after some starlings in the barn yard... I popped 7 between 30 and 60 yards.. I thought pellets made a loud pop....but slugs...WOW!! they hit hard!
 
I'm glad you found your magic dragon! 🐉 Another thing that you might consider is that the slugs have a larger surface area contact than the pellets, and they may start to slow down before they even leave the barrel with your current pellet tune.

After you run some ballistic calculations, you may find the slugs flying 30% faster at 60 yards than the pellets, which may be the reason for the extra loud pop! I shot a Starling with a .25 cal NSA slug using my thermal scope at 40 yards, and I saw his entire head pop off! I got video if you are interested. 😃