You can always go UP in caliber for a moderator.

Downside - It might not be as quiet as a moderator sized exactly to the caliber of the gun. But it might also be that you don't actually hear much difference.

Upside - It will provide sound moderation with a lower possibility of pellet clipping/accuracy issues as compared to a moderator sized exactly to the caliber of the gun. And pellet clipping/accuracy issues may be caused by 2 different things:

1) The pellet physically striking the internal components of the moderator; or

2) The pellet's flight being impacted by turbulent air at the muzzle/outlet of the suppressor.

With a 'sized up' moderator, there is a lower potential of either of these bad things happening.
 
I was thinking tha some were talking %$×@#*&^ about the most popular brand LDC.. recently treaded my 2 texan barre M-14×1.5 l and said to the machinists .to make the treads a tight fit to the ldc ..he show me the tooling n I se for my self the wobbly problem is the over size fit in the treads..not his work..futher more I purchase this week adapters from US to install muzzle breaks .. no wobble at all nice tight fit...so the clipping problems are really bad treads...
 
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.