If you are a reloader or know one, get a couple of loose bullets and drop down the aperture. if they go, they can, if they don't, don't.
much more reliable than a ruler.
Don't forget it's the aperture inside the moderator you need to measure, the stainless steel bit, not the aperture at the exit end.
I had this problem with my t8 to check the size I used a box of metal work drills which move up in 0.5mm steps just slide one in till you get a good fit (as the actress said to the bishop). Out of interest my t8 would only go up to .243 and I have a wildcat on my .270
Well I went and had another look at it, ruler in hand. The dimension of the exit aperture was ~7.5 mm and it was the same through the baffles, so I bought it.
Took it down to the farm, set up a target at approx 100m, loaded up a single round and fired a sighter. The good news was that I did not launch something resembling a baton round, and as it was the first time I had ever used a rifle with a moderator I was suitably impressed with both the sound reduction and the recoil reduction...
But, as put a few more rounds through it my normal quite poor grouping degenerated into quite appalling grouping, I would be lucky to hit a 6" circle consistently. My brother was with me and fired just two shots, both into the 2" bull and declared that there was nowt wrong with the set up and had I considered taking up golf?
Anyway I took the moderator off and shot an acceptable group with the next 3 shots and left it at that before I demolished what little bit of confidence I had left.
Fitting a moderator should improve your shooting, not make it worse! First thing to check might be that the mod is threaded the same as your muzzle. I don't use a Wildcat myself but if it is anything like the Reflex there is a plastic bushing at the lower (breech) end of the mod - on the Reflex this needs to be bored to fit your barrel for maximum efficiency and yours may need replacing with something more suitable. Time to visit a decent gunsmith, I suggest .......
Also - if the mod is proofed & stamped for .223, I'd be very reluctant to put anything larger through it whatever people say!
this might be a silly question but did you let it cool between shots.
In my experience moderators make the barrel heat up much quicker especially in larger calibres try shooting a two shot group remove the moderator for ten minutes let it all cool and shoot another two.
My 300wsm was very accurate but after four shots the group would open up dramatically.
Hope this is some help.
i recently bought a predator 8 for my sako 85 in .243
noise reduction to something like a moderated rimfire
recoil reduction " " " "
however.. i notice after about 20 consecutive shots into the paper targets ( i was trying out different ammo) that i started to get some optical interferance.
at 1st i thought it was some grass obscuring my line of sight but then when i touched the sound mod it was too hot to the normal touch.
i believe that i was experiancing "heat waves" from the barrel it was consistant with the interferance.
so like as been said before after 3 or 4 shots take the thing off for 10 minutes to cool down as it acts like a big fat sexy heat conductor.