Just like any other law..
The residency requirements for MO are now 90 days, they did indeed used to be six months.
Now, you can use your shiny new license to purchase a shiny new firearm, however, the recent move could trigger a 4473 delay, but those are only five days long or five very long days since you really want to go shoot the new toy!
Residency restrictions are simply one of the hundreds of regulations a gun dealer must comply with, there are changes every year, does not make him a bad dealer, just one who missed the change in one law a few years ago, a law that would not be monitored by gun advocates etc. A divorce lawyer would be more likely to monitor such things as OOS divorce is an often employed strategy to make it harder on the opposing party. If the ex-to-be is in OK city and you move to St. Louis, wait your 90 days plus an additional 30, you can then file in MO for divorce, if you do it before then, it is kicked back because of jurisdictional issues and OK state is where you must file, blather blather etc.
Residency laws vary by state, more have moved to the 90 day thing than used to be as it caused significant issues for things like divorce court. You also must watch things like the old state permit requirements etc. Say you moved from state X and they said as soon as you move your permit expires, you need to remember that and until you have your 90 days in MO where you can swap everything you need to be "living in X but moving to MO" if pulled over by the popo so you do not put yourself in to a court room mess for concealed carry without a good permit. you would likely win etc, but $$$$$ marks the spot.
Food for thought as residency laws are more of a pain in the butt than they are worth.
