When I bought my .177 Avenger from Donnie of Baker Firearms, It was recommend that I go with a .22 cal moderator (one size up) So, I got a .22 cal DonnyFL TANTO moderator (along with a DonnyFL Avenger adapter so I could put it on) and all is well.
Later, when I bought my .25 Daystate Wolverine HP R, I emailed DonnyFL with the same question on my mind, Donny said to go with a .25 mod (the same size as the gun), said it would be fine. I did and all is well, too. Shooting without it and then with it, is like night and day. It just screws on, no adapter is needed. And it never comes off.
So, I guess if you buy a quality moderator, you can buy the same size as your gun. I'm thinking that when I have some spare $$$, I'll go back to DonnyFL and buy a .177 TANTO - but I'm afraid I'll be wasting my $$$ because the .22 TANTO may be just as quiet as a .177 TANTO.