I'm not sure the coil spring powered hammers could be converted to rebounding type. But the hard opening after firing is a sign that there is wear/slack in the cocking linkage.
On the 311 style action it's usually the Cocking Plunger (located in the bbl lug itself) that is not in very close contact with the Cocking Lever (located in the frame).
That lack of contact or extremely close contact allows for the bbls when unlatched after firing to have to drop some distance before that slack is taken up and tthe hammer(s) start to move back and off of the firing pin(s).
Having to drop the bbls any distance with one or both of the fired Firing Pins imbedded into the fired primers is what you are fighting against when trying to open the gun.
You can check for any wear or slack in the linkage parts by
First: Make sure the gun is UNLOADED.
Then close it and dry fire one or both bbls.
Then with it still closed, hold it horizontal out in front of you. One hand supporting it by the forend. The other to unlatch the thumbpiece.
With the thumbpiece pushed open,,slowly let the bbls drop open by their own weight,,still supported by you hand.
They will likely drop down a 1/16" and more and then come to some resistance.
At that point, the cocking linkage has now taken up any wear or slack in itself.
Look at the amount the bbls have opened already,,how much the top edge of the bbl is now above the top edge of the frame edge.
If there had been shells in the gun and instead of a dry fire experiment, you had live fired the gun,,you would have had to drag the open that far with the fired hammers down all the way
under spring tension upon the firing pins imbedded into the primers.
Only when the bbls have reached the point where they stopped in this experiment when you feel the linkage take up the slack are the hammers just beginning to be retracted.
The 311/ModB can usually be 'fixed' by repairing the 'Cocking Plunger' (mentioned above).
It's the chisel nosed rod extending out from the bbl lug back towards the frame.
That reduced area nose is pushed underneath the Cocking Lever when the forend is placed on the gun when assembled.
When the bbl are opened, the Plunger lifts the Cocking Lever upwards which cocks the hammers.
There is usually plenty of space betw the Plunger surface and the Lever surface,,that is where the 'slack' exists most often.
Sometimes you can heat the Plunger end (red) and carefully bend it upward a bit and that'll fix your problem. Some extra fitting afterwards may be needed.
Or you can add matrl to build up the point so it makes contact (or nearly so). You have to experiment to get a feel for how much thickness you need to add.
Shims glued in place, tape, even clay stuck to the surface of the Plunger and then the bbls placed onto the frame, then the forend put on & taken right back off can work in giving you a good idea of the amt
of space betw the 2 parts that needs to be built up.
Or you can simply soft or hard solder a steel shim in place and fit it by using smoke or spotting compound.
There needs to be a tiny bit of clearance betw the Plunger and the Lever when they are engaged with the Forend in place. You do not want them to get jammed together.
They must be able to disengage when the Forend is taken off. The Plunger is springloaded and that is what will pull the Plunger back out of engagement with the Cocking Lever and allow the
BBls to be taken off the frame.
