Agreed
The only reason I was thinking a final lapping may be neccessary is that nitriding has, traditionally been known to ever so slightly roughen the surface finish of the substrate.
Bear in mind that ever so slightly in this case means fractions of a microm to perhapse a couple of microns rougher.
Considering the lapping abrasives used for hand lapping the bore of even the finest rifle barrels are probably significantly rougher than this, I'm not so sure it'll make any difference, but I would maybe run some flitz, jb, or simichrome down the bore on a tight fitting patch.
Do not worry about damage to such a bore from these semi abrasive cleaning / polishing agents. You'd really have to try pretty darned hard to do abrasive damage to a nitrided surface with abrasives this fine.
As for the chamber, I would vehemently reccomend AGAINST nitriding the chamber. Luckily enough, the bore can still be nitrided after the chamber is cut, as masking off this area is not any big deal, however, nitriding has the added benefit of decreasing the coeeficient of sliding friction of the steel against other metals.
While this lower coefficient of sliding friction will mean lower pressure from the same loading (thus lower velocity from the same podew charge) it will also mean more powder can be loaded to make up for this loss of velocity while maintaining safe pressure levels, and even a gain in the untimate velocity potential of the cartridge (assuming current load density is below 100%).
However, this decrease in sliding friction will work against you in the chamber area, resulting in much greater stress on the bolt face, and possibly premature failure of critical components, creating an unsafe operating condition. Thus the chamber should not be nitrided. The throat would be ok though.
Nitriding a barrel that isn't properly broken in could be a pain though. When the chamber is cut, tiny ridges at the head of the throat left by the reamer is the most common explanation I see for break in. If those tooling marks are nitrided, breaking in the barrel may take a hundred or more rounds!
I see two possible soloutions to that problem:
1 - I have heard of some people who are chambering with wire EDM. This would eliminate the problem entirely.
2 - in my "why not" thread, I proposed an idea that could solve this problem as well. Chamber after gun drilling, before the rifling is cut. Then, machine a "chamber plug" that exactly fits the chamber. With an oversized "cap" on this chamber plug, it could be threaded onto the breech of the barrel. Gun drill this plug to fit as closely and seamlessly as tolerances will allow. Then, the rifling is cut into the bore, and continued into the chamber plug. This way, when you hand lap the bore, you can run the lap down into the chamber plug, allowing you to lap the throat right along woth the rest of the bore, and the "funneling" or bell shaped widening at the point where the lap is reversed is in the chamber plug, and not in the bore itself. This creates a perfectly lapped bore, with no cross directional tooling marks at the throat from the chambering, meaning no break in for the barrel!
The only reason I was thinking a final lapping may be neccessary is that nitriding has, traditionally been known to ever so slightly roughen the surface finish of the substrate.
Bear in mind that ever so slightly in this case means fractions of a microm to perhapse a couple of microns rougher.
Considering the lapping abrasives used for hand lapping the bore of even the finest rifle barrels are probably significantly rougher than this, I'm not so sure it'll make any difference, but I would maybe run some flitz, jb, or simichrome down the bore on a tight fitting patch.
Do not worry about damage to such a bore from these semi abrasive cleaning / polishing agents. You'd really have to try pretty darned hard to do abrasive damage to a nitrided surface with abrasives this fine.
As for the chamber, I would vehemently reccomend AGAINST nitriding the chamber. Luckily enough, the bore can still be nitrided after the chamber is cut, as masking off this area is not any big deal, however, nitriding has the added benefit of decreasing the coeeficient of sliding friction of the steel against other metals.
While this lower coefficient of sliding friction will mean lower pressure from the same loading (thus lower velocity from the same podew charge) it will also mean more powder can be loaded to make up for this loss of velocity while maintaining safe pressure levels, and even a gain in the untimate velocity potential of the cartridge (assuming current load density is below 100%).
However, this decrease in sliding friction will work against you in the chamber area, resulting in much greater stress on the bolt face, and possibly premature failure of critical components, creating an unsafe operating condition. Thus the chamber should not be nitrided. The throat would be ok though.
Nitriding a barrel that isn't properly broken in could be a pain though. When the chamber is cut, tiny ridges at the head of the throat left by the reamer is the most common explanation I see for break in. If those tooling marks are nitrided, breaking in the barrel may take a hundred or more rounds!
I see two possible soloutions to that problem:
1 - I have heard of some people who are chambering with wire EDM. This would eliminate the problem entirely.
2 - in my "why not" thread, I proposed an idea that could solve this problem as well. Chamber after gun drilling, before the rifling is cut. Then, machine a "chamber plug" that exactly fits the chamber. With an oversized "cap" on this chamber plug, it could be threaded onto the breech of the barrel. Gun drill this plug to fit as closely and seamlessly as tolerances will allow. Then, the rifling is cut into the bore, and continued into the chamber plug. This way, when you hand lap the bore, you can run the lap down into the chamber plug, allowing you to lap the throat right along woth the rest of the bore, and the "funneling" or bell shaped widening at the point where the lap is reversed is in the chamber plug, and not in the bore itself. This creates a perfectly lapped bore, with no cross directional tooling marks at the throat from the chambering, meaning no break in for the barrel!
