seems to me if the statuate sattes a 'case' max length.. then that is what will prevail.
kinda like having a shotgun that hods 6 rounds with plug removed.. but 3 with it in.. and needing plug in / 3 shot to be legal to hunt. proof is in the pudding.
IE.. your loaded cases in your pocket or magazine.. if measured .. and trimmed to 1.8 seems that is it.. can't go off the spent cases as they grow at time of shooting.. so must go off loaded cases.
many firearms have a chamber that would accept a couple different cartridges.. so simply having the chamber marking as the default length is not a good test.
my rifle chambered for 458 lott also safely chambers 458winmag. a shorter cartridge. In fact. i can't even find any 458 lott locally.. so i've ONLY been shooting 458 win mag in it so far..
357 chamber shooting 38spl.. etc.
for cartridges spacing off a rim.. look at a judge and 45lc and 410 shotgun.. same cyl /chamber.. 2 drastically differnt cartridges.
for chambers that space off the bottleneck.. the cartridge throat being a hair shy ahead of the necking, as others stated shouldn't be an issue.. especially if you can keep loaded COAL the same so lead is the same to rifeling..
it may be a hassle.. but AFAIK, burden of proof is still on the state. and having a handfulle of cases that meet the laws definition seem to be pretty potent defense evidence to me?