here's an update..
so I was told by CSA that I had a problem rifle, so I sent my rifle in to Movie Arms.
I just got my rifle back.
1. I measured the size of the returned piston with the one from a CZ858 - it has the same diameter head!
2. There is a gouge on the side of my NEA handguard (bottom)
3. the spring and bearing that holds the upper handguard pin in are missing!! In other words the pin can be freely removed and is no longer attached to the upper handguard.
4. they did not send me any reimbursement for my shipping it to them
5. they did include a work order. They said they examined the gas head, replaced the piston, fired 20 test shots and then cleaned it.
So the reason why I sent the rifle in, was that CSA told me (based on my serial number) that my gas hole was 3mm rather than 2.5mm like it should be. So I decided to check it when I got it back since the piston looked exactly the same size as a stock CZ 858 piston. I got two drill bits, one slightly over 2.5mm and one slightly under 2.5mm. The smaller one fit inside the gas hole no problem, the larger one did not. So it appears at least from my test that my gas hole was 2.5mm after all??
Yes I should probably have checked all this in the first place, but I only just thought of the drill bit idea, and because the gas hole is drilled at an angle it does look at least 3mm large. Anyway, if I get my shipping reimbursed, I'll call it a day (I have a spare spring and pin holder).