I appreciate the questions.
There's a lot more wood to come off. In times past, I would hog off almost whatever needed to go, but lately, I've been prone to leave as much "square" and overdone as possible until the last minute. The square faces and edges help with any jigs I design to do whatever voodoo I need to do.
All my reference lines have been sanded off, as I "sketched" so much on it, that I cleaned it up to regroup. Luckily, everything along the forearm is symmetrical and I have some small reference nicks I use to rescribe where needed.
The lock is at the right spot. The hammer goes into a groove in the stock that is normally relieved a whole lot more on the underhammers, as well as another eighth or so at the front of the lock towards the muzzle that needs to come off the bottom.
I'm using a long threaded nipple as well; given the straight ignition, the extra length doesn't scare me. I have metal stock on hand to devise a "spacer" that will hold the nipple at the appropriate depth, and this spacer may double as a flash guard as well.
If I can make it work, the hammer tab will project just enough from under the stock to get a grip on to cock it. And capping will be done with the straightline capper (that I am gently bending) along a "ramp" in the center of the forearm. I'm using the defunct stock to test this plan. If it don't happen, then I'll relieve enough wood to make it happen as traditionally seen.
Just working out if the flash guard, ramrod end, and the nipple location to one another will allow this to happen.
Right now, the web between the ramrod hole and barrel channel is an even 3/16 throughout the forearm. It does fade some to the muzzle end, to 1/4" - I can live with that, but I drilled a couple holes this weekend to really check that web in the forearm area and its 3/16". It is my intent to have the RR stop at the flash guard.
I will leave it full stocked this time, all the way, like a longrifle stock as you referenced. Thinking of an entry pipe at the end of the forearm, than a couple pipes in the fore-end between the entry pipe and nose cap.
Better pics would help, for sure, but my daughter has my wife's camera, and my cell phone sucks for detail.
A sketch or marked up picture would help as well.
If I can download all the bird pics my girl has taken, I'll try to get something visually understandable posted.
