Two possible answers- first, it could have been removed by grinding or filing it- a VERY illegal thing to do.
Second- and very likely if it is an older rifle or shotgun- it never HAD a serial number. They were not required by law for rifles and shotguns until 1968. If made prior to that date, it never had a serial number (and yes, they are legal if they never had one)
It is a four digit number on the receiver under the right side barrel, open the gun and look deep on the receiver,
It depends on what you want to find out.
You find a published list of serial numbers for the maker.
Missouri does not have gun registration!
Find books on the maker.
you will find it on the barrel or action of the firearm
Serial number and/or features.
No
By the serial #
serial number 28402
Some are easier to research than others. For example there is a Remington Collectors Society, but you cant find much information on Remington serial numbers. On the other hand if you have a Winchester serial number you can usually find when it was made without much difficulty. Google for the make of gun you have + collectors. Some of the manufacturers will research the gun, for a fee, and send you a Factory Letter, stating when the gun was made, and where it was sent from the factory.
On line auctions, gun shops, gun stores, pawn shops