Every site should Chrono, no if or buts its an absolute MUST!
What i would propose is that each site Chrono at the start of the day every players gun (1)
By every gun i mean AEGs, DMRs, Bolt action Rifles and PISTOLs (2)
Use a sites magazine preloaded with a known weight, either 0.20g or 0.25g and the Chrono matched to those weights (3)
Use the players preloaded magazine with their weight which they must declare, it should fall within a certain speed on the chrono (4)
Use Joules NOT FPS or MPS (5)
1: Every player needs to have their gun chrono'd, regardless of whether they are a regular or newbie, using their own gun or a rental, this includes marshals or staff who want to play the occasional game.
2: I mean every gun to be used that day or potentially used, i included Pistols as some people field CO2 Pistols which can occasionally be way too hot for Airsoft play.
3: If a site uses its own preloaded mags in popular mags such as STANAGs, AKs, G36 etc with the sites known weight then this reduces the chance of people going up to the Marshals and saying "yeah i got 0.20g BBs loaded" when in actual fact they have 0.30g BBs and hitting an FPS of 345 which would put them shooting at 1.6joules (around most sites DMR limits)
4: Chrono the gun again using the players declared weight, if the player says he or she is going to be using 0.25g BBs and if their gun clocked in at 345FPS on .20g BBs then it should clock in at around 310FPS on 0.25g BBs, this is to prevent Joule creep as you go up the BB weights.
5: Use Joule and NOT FPS or MPS as in reality what the UK legislation regarding Airsoft guns speaks in Joules not FPS.
6: Each gun to be chrono'd must have its Hop Up set to the lowest setting.
I'll admit on points 3 and 4 you may have a problem with having a variety of too many magazines or some players turning up with weird or hard to find mags such as a Kriss Vector Mag, if thats the case the site should have a speed loader with a known weight of BB ready, the player should provide an empty mag of theirs, which should be loaded up with that BB weigh and tested.
Furthermore, to prevent any discrepancy all sites should use the same Chrono brand and make, most of the sites i have visited use the Xcoretech X3200 and whilst i perfectly understand that not all chronos are made equal even among the same make it helps to reduce discrepancy. Some of my guns are shown just below the 350FPS limit on Xcoretech but on others such as Madbulls is shown around 325FPS (what gives).
All my above points is to try and prevent players from cheating the Chrono.
As for HPA setups, definitely use tournament locks with serial number zip ties, what prevents a HPA user from arriving to the site with a handful of coloured zip ties, get chrono'd with a legit FPS, get tied by staff, goes to the safe zone, removes the staffs tie, turns up the regulator and puts his own tie of similar colour through the regulator as if to say that it was chrono'd ok.