JKR,

Two questions that might help you diagnose:

Is there a faint firing pin imprint on the primer? 

Does the hammer fall only to the half-cock notch?

Those answers will help you determine whether the half-cock is catching the hammer.

The fact that you haven't had misfires with the pistol primers might indicate that you have insufficient striker force.  I once had a rimfire Winder musket that had the firing pin shortened to just under 0.040" protrusion.  A new coil spring didn't solve the problem, but a new firing pin with 0.055" protrusion solved the problem.

The Needier firing pin may just need a cleanup to get out anything that may have become trapped, or just a very light coat of oil to minimize friction.

CADyk

I found years ago that the inside of a Winchester single shot can trap a good deal of crud. Old lube, bits of dirt and even other stuff. My first rifle came to me due to the owner having similar trouble as you are having, but not with the Neidner pin no a fly because it is a plain trigger gun. I learned how to take it down to do a 'new gun cleaning' and was surprised to see how much was in the block- no doubt slowing the pin to prevent firing the primer. I wonder if that had ever been mucked out since the rifle was new before the turn of the century. Once cleaned and lube it soon became my favorite rifle. 
One very good feature of the original rifles over those with the so-called Neidner variety is that the pin can be removed without dismantling the whole damn action and emergency cleaning can be done in the field with nothing beyond a screwdriver and a couple of toothpicks. and oil.

Robert,
You might have it!
Your reply sent me running to the shop. With the block closed I can slip a .007 feeler gauge between the breech face and the barrel. Comparing this to an original Winchester I have it’s considerably less gap. But… When I push an empty case tight into the chamber I can see that it’s just a hair deeper than the barrel face. 
I pushed a case in tight, closed the breech, then carefully slid a rod in from the muzzle until it stopped against the inside of the empty case. When I pushed on the rod I could feel and hear the case pop back against the breech face. This is a minute amount of movement that I wouldn’t have noticed with a loaded round because the bullet against the lands hold the case to the rear.  The question now is, is the excessive headspace and causing the misfires? 
To answer your first question. I’ve owned and used the rifle for two years. Over this period I’ve had a few, not many, misfires. Lately the misfires are increasing. I shot a ten shot string plus sighters yesterday and had five. 
Thanks for your thoughts on this. 
Jim

calledflyer wrote yesterday at 11:45am:


Jim, I'll bet it is a problem with the Mann/Niedner That was a problem with Tom's a few years ago and I fixed his by removing it, cleaning it and polishing the parts and putting it back together and it's been working fine since. I haven't seen a Meacham. Did he use a true Mann/Niedner system where a faceplate is held in with two small screws? I can't imagine the headspace changing because of firing black powder in your rifle.  A .007" gap isn't excessive but I would like mine to be a little tighter than that. You do say that the rim sits beneath the end of the barrel so that could cause excessive head space. Have you changed your brass lately and shooting a different brand?  Are you shooting it this weekend? Bring it along as Lynn and I will both be there and we'll take a look at it. We both know our high walls fairly well and should be able to come up with some kind of answer. I have some stretched Remington 45-70 cases that I will bring along as they may have a thicker rim and were probably used back when your rifle was made.

0bobw wrote yesterday at 4:06pm:
 
Agree, I have a couple of my rifles made up with about .003" headspace and have trimmed the rims of all my brass so that they are the same thickness which keeps headspace down to a minimum. I once owned a Cody Ballard high wall that would take a .012" feeler gauge between the end of the barrel and the breech block. Didn't seem to be a problem but it did bug me.

Bob,
I was hoping you’d see this! I will bring it along. The FP is easily removed by unscrewing the bushing at the rear of the breech block. Near as I can measure the Starline case seats .005” beneath the end of the barrel. Rim thickness on the case is .064”. 
Jim