Here's a schematic of the 311
Stevens Shotgun Parts. Stevens Autsomatic Shotgun Parts, Stevens ...
Part #35 ....cocking plunger..located in the bbl assembly itself.
Part #29....cocking lever or arm....this is located in the frame itself. You can see part of it,,the front leg,,sticking out slightly from the cut out in the frame w/the bbls removed.
The cocking plunger (35) is pushed to the rear and engages the cocking lever (29) ONLY when the forend is in place on the gun.
With the forend removed,,the spring loaded cocking rod moves out of engagement and allows the bbls to be taken off the frame.
Check to see that the cocking plunger #35 is in place. That it is under spring tension and moves forward (muzzle) when pressure is taken off of it. The slight 'flat' on the cocking plunger should be facing upwards and to the rear when in place.
It has to be able to slide underneath the cocking lever in the frame to rotate the hammers back and cock them to the sears.
The forend iron itself is what pushes the cocking plunger to the rear and into engagement.
You can check to see if it is with the bbls off of the frame. Assemble the forend (complete) onto the separated bbls. Carefully set it in place as if the frame were still attached and rotate it down to the bbls and snapped shut. It doesn't need the frame to be held in place.
With the forend snapped onto the bbl assembly, watch and check to see that the cocking plunger moves backward under spring tension as it should. It should move forward as the forend is then removed.
I think there are 2 different diameter, slightly different cocking plungers (35) for the 311 series. Could be a problem if the smaller dia one is in place where the larger one is supposed to be.
The front leg of the cocking lever (29) being completely broken off is a possibility, but a rare one.
Nothing for the plunger to engage at that point so the hammers don't retract when the bbls are opened.
Short of something in this sequence not set up right, worn, missing or damaged causing the problem,,I'd have to have it in hand to see anything further.
Hope this helps.