Are (Zbroia) PCPs Moderator Picky ?

Yeah anything attached to the barrel will influence its harmonics, and there's no way to predict whether it will be favorable or unfavorable or indistinguishable to group size. Fortunately my experience has been that it's usually either favorable or makes no noticeable difference, but on occasion it will make groups worse.

But the fact you've had a similar experience with two different moderators, I would take a close look at parts and features in proximity to the muzzle (the end of the barrel, not the end of the shroud). For example if the muzzle is loosely touching a spacer that keeps the barrel centered in the shroud, the vibration impulse from firing can make the muzzle vibrate. Think about two metal parts that are almost touching...tap one of them and suddenly the two parts are buzzing. Vibration of this sort at the muzzle, even on a small scale of a few thousandths of an inch can produce a much larger dispersion at a distance. If you find a little slop between the parts--and you will--try shimming them with clear tape until they are a slip fit and then try some groups again.

Harmonics are key, as well as stripping the air away from the pellet as it leaves the barrel. In my testing some moderators actually improve accuracy on one gun but not on others. 

Yes.

I had a Rocker LDC and I have a TKO LDC for my 1720T

The TKO is perfect.

The pistol stacks the pellets with or without it.

The Rocker changes the POI.

But, if I adjust the zero on the scope, the Rocker is fine.

often the main problem with shrouded barrels: many moderators are to heavy

they sometimes pulling down the shroud and misaligned the shroud to the barrel

a very good example is the dreamline shroud. its very weak in design: there is a o-ring holding it in place in front of the action. o-rings are very easy to flatten out.

This has always intrigued me the harmonics of airgun barrels, shrouded, unshrouded, tensioned vs. compressed etc.

This is what I've found, first it takes many groups to know if one or the other is truly effecting group size, as many variables as possible need to be eliminated as you test! ( you should use be using wind flags to eliminate that variable as well ) 

The FX's I've owned seem to want the moderator, the Daystates, two didn't one did, the RAW's don't care, The AAA I didn't even bother to try expecting no difference!

Hoping some one like motorhead will chime in or other tuner's, Maybe manufactures Mike of Thomas or Tom of AAA?

The groups are vastly different in size, disappointing for sure. My latest PCP is unbearable without a moderator. 

I'd be inclined to experiment with a variety of ammo and inspect the end of said moderator with magnifying glass for scraping before moving on.

Don't know if this would help, but I put teflon tape on the adapter threads. It may limit the metal to metal contact some...but I've never tested the theory, so it may do nothing 

Don't know if this would help, but I put teflon tape on the adapter threads. It may limit the metal to metal contact some...but I've never tested the theory, so it may do nothing

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I also thought of doing that. I am going to try that. Seems like a good place to start.

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Who knows? Couldn't hurt to try

I drilled my Tatsu 22 cal out to 25 cal, problem went away. to many threaded connections, each connection has a slight error, it was drilled larger by a machinist, not by hand,

Does the Kozak FC have a tensioned barrel? the Kozaks (slow joe calls them tactical) do have the baffles in the shroud that pull tension on the barrel and push on the shroud, I don't know about the FC(fully compact) models. The amount of tension does affect accuracy

Donny often recommends to buy the next size larger

did you try swapping moderators with the Prod? just a thought.

you can play with the tension, start with a little tension, shoot a group, tighten a little more shoot another group, I read the instructions on a shampoo bottle, tighten a little and repeat

I am the kind of guy that would have a look inside of the problem moderator, are all the baffles in line, or should I say, are all the holes in line, it could be just that moderator, not the brand or model, just a defect in that one. could be one of the threaded connections is off a hair, causing a misalignment. It would be interesting to pull the barrel with the mod on it and have a look through it> Best of luck figuring it out, keep us posted

but the fact remains, gun shoots great, install said mod, gun shoots scattered, I am leaning towards the mod as the cause

I have a Sumo and Tatsu. Both in .25 cal. I shoot them one my .22 caliber Zbroia Kozak FC 550. I have found the shroud tightened to a specific spot, makes a difference. Maybe it's in my head, maybe not. Either way, I marked the receiver and the barrel shroud to line up to a repeatable tightness.

Both moderators improve my group size (tighter with the moderator on)

This is a great 100 yard group for me, but certainly repeatable.

Do you have the ability to adjust the hammer spring easily? Maybe try a adjusting the HS to see if that changes the group size after your moderator install. Just a suggestion. Im no expert. With help from forum members here, I got over my fear of screwing everything up, and started adjusting HS for different pellets and slugs. Wow, what a difference. When u get the HS dialed in, the guns are dead nuts on. Damn near hole in hole at 50 yards. This gun shoots better than I am capable of, and certainly better than any of my powder burners.

Maybe try a different pellet or slug. Each barrel is different, but mine loves the Air Arms pellets and the FX Slugs at different HS settings.

Don't give up on it! And don't be scared of trying some adjustment. These guns are well built. If I can do it, you can too.

Also, the ring that the barrel shroud tightens to can move a bit if I recall correctly. That could make it appear as if it's where you had it last, but it's actually different.