Too wide of a slug, obviously. That's why there are so many different weights & diameters of slugs available. The rabbit hole gets deep trying to find what works best in our idiosyncratic "toys". There's really no way around the buy & try method until one finds what works.
Clearly
The avenger has a tight barrel
216 diameter slugs should work.as a starting point.
Remove barrel if poss and see if you can push slug through by hand ' you shouldn't need excessive force and look how deep thatrifling marks are on the slug
dont try and force an fx slug down it.
Hope this helps
il try that method.
 
Certain 22 cal slugs will get wedged in the avenger barrel as its tighter than most guns for some reason.
But it also makes it accurate.
A 218 will not go through a 216 does and even that might be on the big side.
Hence start at 216 and go smaller if required samo.
Dont expect stunning results either first time round' you need to test different diameters and grain ' to find what works for your gun.
The avenger can shoot a slug well.
You shouldn't have to force the cocking arm at all why loading " but I also know from experiencing it myself the bigger slug will load quite normal but then refuse to leave the barrel.
Hope you find this helpful