$1,000 or so depending upon condition and point of sale.
On the bottom of the barrel, next to the forearm.
look under the forearm
Check the receiver, barrel, under the forearm
The same size forearm is used on all barrel lengths of the 500.
Go to the Browning link attached to this answer. Be sure to print it out and save the drawing. Many people glue the drawing on the inside of their forearm where it won't hinder the barrel and recoil spring.
remove pin from end of magazine tube at barrel muzzle, pull magazine tube from forearm, forearm is now ready to be lifted off barrel
The one my dad has uses the same forearm. It should just snap on just like it does on the rifle barrel.
First of all - the breech must be empty and open. Under and forward of the reciever is a release lever. Pull the lever back toward the stock. The forearm will pivot down - press the forearm down further with your thumbs on each side. The forearm will clunk when it is pressed down to the release position. You may need to use some force - dont be shy - press it down firmly, it wont break. The barrel will now come easily away by pulling it forward away from the stock. To replace the barrel, again make sure the breech is empty and open - then slide the barrel inward to the breech (a slight angle downwards into the breech makes it easier), make sure you press the front locking cleat on the underside of the barrel into place locking it in the forearm. When seated all the way in, use the palm of your hand under the forearm to close the forearm firmly until the release lever snaps into place locking the forearm. Give it a decent and firm slap if required.
The forearm is the front part of the stock, under the barrel, that your NON-trigger hand holds.
Although both are interchangeable, a true Sweet 16 barrel will have three holes drilled through the barrel band. This was one of the ways Browning lightened the barrel. The rib will also be a smidge thinner than on a regular barrel.
You will have to ask Browning.
First, unscrew the magazine tube cap, located at the end of the forearm. Then pull the action back to about half-way, then you should be able to pull and jiggle the barrel and forearm free. To reassemble, slide the barrel and forearm back on, and pull the action back until the barrel slides into place, then rescrew the magazine tube cap.