I haven't seen one of these taken apart in 30 or 40 years. No wonder I don't remember how to take one apart. My, how the years go by. John (edited for spelling)
www.wisnersinc.com/additionalinfo/winchesterl_88_100.htm
MODEL 100 DISASSEMBLY: The proper disassembly of the model 100 is to first remove the magazine, then remove the forearm screw AND the trigger guard screw behind the trigger guard. Now pull the operating handle all the way to the rear. While holding it to the rear, pivot the barrel unit up out of the stock. The rear of the receiver is held in place by it fitting in a notch in the recoil block in the stock. The reason that the operating handle has to be rearward is to allow enough clearance for the side rods from the handle to the gas cylinder unit to clear the slot in the forearm section. Since these rods are hidden in the forearm with wood covering them except when the handle is pulled all the way back.
If the gas cylinder is rusted so tightly into the operating slide assembly that it can not be pulled rearward, there is one solution. You will by now have some movement of the barrel away from the forearm. Select a long punch that will go in between the barrel & the forearm. Locate the small retainer ring that locks the gas cylinder sleeve/plug in place, hit it hard enough with the punch to break it. Now you can use a large long screwdriver to engage the slot on the end of this plug, unscrew it. You should now be able to move the operating handle to the rear. What you have done is to remove the “cap nut” that holds the gas cylinder into the barrels gas lug, the cylinder being rusted in the operating handle, it will now be retracted out of the lug, allowing the handle to be retracted. This information supplied by a long-time Winchester warranty gunsmith Walter Lodewick.
MODEL 100 OPERATING HANDLE FREE BUT NOT RETRACTING ALL THE WAY :- This usually can be attributed to the owner trying to disassemble the gun using the thought that the trigger housing will come out first from the bottom. They can not get it to come out (usually bending the side rails) so they reassemble it, only to now find that the handle will not operate. What they have done is that the safety somehow got pushed off, in the process, they pulled the trigger & not knowing what has just happened, when they retightened this unit the hammer’s top rear corner is now bound up between the bolt sleeve & the head on the rear of the firing pin. The trick is now to remove the rear guard screw, pry the trigger guard assembly down far enough to reach inside & recock the hammer with a long hooked rod.