Rti Prophet .22 or .25

I got a compact prophet II 510 mm barrel or 20" Its got the 20" shroud the barrel comes right to the end and it is LOUD , 22 caliber, I have a Tatsu moderator on it and its still the loudest gun I own, I shot it a few times with out the moderator on it and the wife came out the back door and ask me what the hell I was shooting, 1st time in 20 years, Its loud. 125 bar/ 18.13gr pellets 980FPS CZ barrel,
Mike
 
What is the recommended diameter slugs for this gun? Might as well order a bunch of ammo too.
I had some H&N .218 21 gr slugs when mine arrived. I slugged the barrel and measured with 2 micrometers on 2 different slugs. One slug measured .216, and the second one measured .2165. So I ordered an assortment of different weight .217 NSA's. Hoping to do some more testing with them when the wind dies down.