Here's one of the best answers I could find on firearms forum. To clean your Remington 7400 Model, follow the following steps. "Remove the magazine and cycle the action. Put safety "on-safe". Drift out the two crosspins in the receiver. Move trigger group forward, down and out. This assembly can be cleaned as a group with aerosols such as Gun Scrubber. Remove the large bolt in the front of the forend, then the forend can be slid forward and off. There is a large roll-pin which secures the action tube to the gas tube housing. Using a roll-pin-punch, drift out the pin. Using a standard type punch will damage the pin. Bring the bolt to the rear and hold it there. Slide the bracket you just drifted the pin out of toward the rear and tip the action tube downward and then forward. Do not release the bolt or you will damage the gas tube. Slowly allow the bolt to move forward as you remove the action tube. You can now remove the recoil spring carefully (it is under tension). The barrel is held on by the large nut at the bottom front of the receiver. Be exceptionally careful to match the wrench to the nut (there are special tools available, and they should be used). The nut removes counter-clockwise as viewed from the front. Remove the forend stabilizer spring mounted behind the nut as well. Move the bolt halfway back to the rear again, then move the barrel forward out of the receiver. Move the bolt back forward, then turn the barrel 90 degrees and remove it from the action bars. The bolt handle is retained by a small cross pin. You will have to position the ejection port cover and bolt just so to see it. It is drifted downward for removal, then remove the bolt handle. This allows the bolt and action bar assembly to be removed through the front of the receiver. When cleaning, pay attention to the gas ports in the barrel (there is a special tool available)".
Be sure to always have the safety on your Remington 7400 first before you try to clean it. Remove the 2 crosspins from the receiver first and then the trigger group. You should then clean these parts with a good gun cleaner. You can find more instructions here: http://www.thefirearmsforum.com/showthread.php?t=378