
Here is a labelled image. Not shown are the Wildcat Predator 12 which is the big one in the image above and three other moderators that were in my safe:

Okay, so what have I found?
These are the rounds that will perform at a hearing safe level through an R55 Benchmark rifle (as in the picture above) with all the moderators seen in the picture:
RWS Target Rifle
RWS Rifle Match
Eley Sport
SK Magazine (in the bulk tin)
CCI Standard
Those all cycle my semi-auto R55.
These also work, but with problems noted:
Aguila Colibri. These are dedicated subsonics and will not cycle
Aguila SSS. These are dedicated subsonics but produce a lot of noise, sparks and flash from a semi-auto action. They also key-hole in all three of my semi-auto rifles. I do not recommend them, not for safety reasons but because they are poor performers.
Another issue is with the last batch of CCI Standard I got. The bullets were not secure enough in the cases and I had a lot of failure to rise in the magazine, failure to leave the magazine and failure to feed. So I'm off those for a while.
Same with Eley Sport (hard case with light strikes a few months back) and SK Magazine, which I have fired 1000 rounds of and had failure to fire in several properly struck cases. But I have fired them on one rifle only so far.
Other rounds I have tried which worked:
Eley Tenex
Eley Subsonic
Fiocchi Subsonic
But for plinking I think your best bet is RWS Target Rifle overall, and possibly SK magazine (I'll report back when I have fired that through a different gun).