Unscrew the magazine tube cap, pull the barrel off to the front, remove the forestock.
If in doubt, consult the owner's manual.
A replacement owners manual is available through Remington's web page.
The 870 Remington shotgun used to be a popular candidate for a "release" trigger.
Remington does not show a 870 rifle on their web site. Do you mean the 870 Shotgun? the Remington 870 is one of the most popular shotguns in the us. this shotgun ranges from $300 for the basic model to $1000 for the fancy Wingmaster edition
Remove the cap from the end of the tube and be careful of the spring which is under pressure. The plug will come out with the cap.
The Remington 870 is a type of shotgun which is known as "pump action". The Remington 870 requires the individual to "pump" the shotgun following each pull of the trigger. It is considered to have a slow rate of fire due to the pump action which is required.
Pump action shotgun
Go to Remington's web site and request an owner's manual.
100-1000 depending on specifics
$350
Pretty cheap. About 300 or a little more.
Remington 870 Express Super Magnum.
Around 300 dollaroos.
The actual magazine tube that connects to the receiver is brazed in there, so it removing it isn't an optimum solution.