The idea behind chronoing with .28s is you get the best foresight of joule creep
It's not perfect, since if they're at the limit with .28s then there's a good chance they'll be over if using .36s, but it's significantly safer than chronoing with .20s and guaranteeing that they'll be over no matter what ammo they use.
I had discussed this with two other admins, there's ALWAYS a way to cheat the chrono, it's just a matter of lengths a person goes through to do it. Regardless, the penalty is the same for any severity of cheating the chrono.
The most surefire way we could come up with was:
-The host has custom, colored orders of BBs; one color for each .25, .28 and maybe .36
-The player buys ammo from the host, chrono's with said ammo, and receives a tag of the same color
IE .28s are yellow, chrono with yellow BBs, get yellow tag.
.36s are red, chrono with red BBs, get red tag.
-Punishment for people not using their color of ammo
-Severe punishment for outside ammo
-Easy to spot check, you know what weight they have, just need to chrono against the max fps for that specific weight. Super easy to catch people abusing the system.
-tamper-proof stickers for PTWs, AEG with quick swap springs, tourny locks for P*s, etc
Problems that arise even with this method:
-P*s have tourny locks to prevent adjustment of fps, however, their control boards are not locked and you could potentially adjust the fps by a LOT just by changing the volume of air used per shot. So like long barreled P*s (which most P* users have a 509mm+ barrel) can use low volume for chrono, then crank up the volume creating joule creep, without ever adjusting the regulator.
-GBBRs, NPAS can be adjusted and the obvious temperature creep issue
-Find me an admin that can spot an AEG with a quick swap spring feature. I've worked on dozens of them and I can't spot them without opening them.
-Someone could go way out of their way to get something like a .36 custom made in yellow, fooling the system.
-Someone could tag their own guns
So really unless you're building and tuning EVERYONE's gun, or strictly using rentals, it's extremely difficult to fully control the fps of everyone on the field.
It's still very much an honor sport, and part of that is being honest with your gun's muzzle energy.