Video which shows Remington 870 bolt disassembly and firing pin removal.

The bolt of the Remington 870 firearm is a mechanical part that is responsible for shielding the rear of the chamber when firing the weapon. That way, the propellant can burn without affecting any of the other unused shells in the magazine tube. Once the shell has been fired, the bolt moves out of the way so that a new shell can enter the chamber. This should be pretty basic knowledge to those who understand the mechanics of how a shotgun works.
When you begin to disassemble the Remington 870 bolt, you must first make sure that all the shells and ammunition have been removed from the chamber and magazine cap of your Remington 870 shotgun. This will ensure that there are no accidental firings as you proceed to disassemble it. After you have verified this, begin to disassemble your weapon by engaging the safety and then sliding the action bar lock in an upward direction. Then, pull back on the forend so that the action is open and then slide back the forend about halfway.

At this point, you will start unscrewing the magazine cap in order to remove it. Be careful that the spring doesn’t fly away after you remove the magazine cap because this can happen very easily. Just slowly decompress the magazine spring as you hold the magazine tube tightly and you shouldn’t have this problem. Next, take out the follower after you take out the magazine spring and then pull the barrel off and away from the receiver. Press down on the left shell latch with your thumb and hold it there. Slide the forend all the way forward so that it comes off the magazine tube.

Now you can begin to knock out the firing pin from the bolt. Once you have done that, take out the pin along with the retractor spring. Reverse the entire process by adding your new firing pin to the bolt and the retractor spring you previously took out. If you have a new retractor spring, then use that instead although it that shouldn’t matter. After you have gotten the firing pin and retractor spring back inside the bolt, reassemble your weapon and then test it out by firing a few rounds at a target.