Excellent discussion. Thanks all. Learning something.
Had not thought of using a teardrop pistol grip for the transition. Comes across very well.
Chuck

Bob, I just want to reiterate how much I appreciate you taking the time to document and explain your work.  It's very informative and I really enjoy following your progress.  Thank you!

Joe

I have followed all your builds and have formed the opinion that you never do anything "wrong" but do occasionally have"correctable oopsies".  This is going to be one impressive rifle when you're done!

Bill Lawrence

Isn't that the mainspring hold down pin? It keeps the mainspring from over traveling or rising too high on Hepburns.

That screw is the main spring stop screw on a Hepburn which allows the hammer to rebound and retract the firing pin.
Some repro Hepburn's have done a slight re-design that eliminates the need for that screw. Not sure about your Walker.
Chuck

Tom, Interesting on the #1 roller having the screw also.

The following is going to really bore some of you.    
I'm going to attempt to explain how this gun rebounds. It is actually very simple but it will take some work to get it adjusted properly in the real world.

When I'm done I would be interested in hearing what everyone think the value of the over travel screw is.  I'm putting it in this gun but still not convinced it is needed as you shall see.

These next pictures are all the related parts, out where they can be seen better, and hopefully I'll be able explain what is happening.

In this first picture I am again showing the hammer in the rebounded position.  You can just see the tip of the spring coming through the front of the trigger.  And the roller is contacting the spring at the rear.

In the second photo the hammer is still in the rebounded position, but is a better picture of the tip of the spring, and as can be seen there is no gap between the spring and hammer.

These first two pictures show the spring pressure between the front and back equalized and holding the hammer in a neutral or rebounded position.

This third picture is shown with the hammer in the fired position. As can be seen the roller has lifted off the spring leaving a gap.  This is probably the reason the stop screw is needed.  I just realized there is lots of pressure on the that spring at this point.

In this last picture, again in the fired position, showing the tip of the spring, at the front of the hammer, applying pressure to the hammer.

So once the hammer hits the firing pin and breech block, the foreword motion stops. Then, the spring tip putting pressure on the front of the hammer, pushes the hammer back until the pressure equalized between the tip and the roller, letting the trigger sear enter the half cock notch and positively holding the hammer in place.

Hopefully this all make sense!
Bob