A little late to this party, but I just finished a bit over six months getting the carcass of what was Montana Rifle Company up and running again - at least the barrel making side.
So on the one hand, I hand lapped my first rifle barrel for cast bullets back in the late 1970's, using printed instructions on how to pour a lead lap, etc as my guide.
And then for the last half or so of this year, I was the ops manager (and learning) the process of making and finishing rifle barrels starting with 12' long bar stock. Troubleshooting the Pratt & Whitney drills/reamers, I looked through bore scopes at lots of just drilled but not reamed barrels, drilled and reamed but not rifled barrels, barrels that were rifled but not yet 'hand lapped' (that means scrubbed with medium steel wool), and finally, finished and inspected barrels ready to ship to the rifle manufacturer who ordered them. Also some custom barrels from high end custom barrel makers to have as comparative examples.
All of them - except for a couple of example barrels from a few of the high end makers - had perpendicular tooling marks at least in the bottom of the grooves.
Machine marks galore from the bore being drilled and drag marks from the button very evident the whole length.
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So for starters, there's a lot of truth in those who say "You'll wish you never used a borescope"... Post Traumatic Borescope Syndrome often occurs". I also believe that in some things borescopes are useful tools. Even after suffering Post Traumatic Borescope Syndrome myself.
Relating to the pic above, the most anal, exacting hand lapping job I did was on the barrel of my pick of the litter 1950 Long Branch No. 4 Mk1* Lee Enfield.
I had my bullet bonus and mortar money burning a hole in my pocket just before I was ready to return home from a Bosnia deployment in the early 1990's - I think it was a Shotgun News that my family sent me in a package of magazines and newspapers when I saw an ad for crates of 10 Long Branch's for sale after being returned from Belgium. I bought two crates; when I got home, I spent my post deployment leave unwinding and cleaning all those rifles of their storage grease, then one by one installing an S&K no gunsmithing mount, then firing a few fouling shots followed by a ten shot group with Greek HXP ball at 300 yards.
I kept the pick of the litter, gave second and third place to my younger brother gun nuts, and sold the rest at a tidy $50 profit each. That sale was the dumbest gun thing I've ever done, looking at what new Long Branches sell for 30 years later.
I digress. I intended to use that rifle for heritage rifle matches, cast bullet association military rifle matches, etc. So I hand lapped that barrel under the watchful eye of my benchrest shooting and hunting and fishing buddy eye gunsmith to what he assured me was as close to perfection as a reasonable person could hope for. Didn't have a borescope back then, nor the WWW. Fidonet, that was it. But it felt as uniform as humanly possible by the end of the last grade of clover compound. It was a long day's work in pursuit of perfection.
That rifle is still so accurate with both the right ball ammo (or handloads) and the right cast bullet that I'd rather not say how accurate because I don't like being called a liar. It was a tiny bit more accurate with the Greek HXP ball after lapping, but now that the scope is permanently off of it and the Parker Hale vernier rear sight is replacing it, shooter error makes it irrelevant. I never tried cast before lapping, so I can't say about that.
And despite a full day of lead lapping to remove any and all tight spots from throat to muzzle, when I finally stuck a borescope in my rifle to look at it years later - there's STILL cross hatch tool marks visible in the bottom of the grooves, none on the top of the lands. Because depth perception through a borescope is hard, I can't say just how deep those marks are, but obviously they aren't causing harm. The barrel doesn't copper foul, and it didn't foul with cast bullets.
The perpendicular tool marks being in the bottom of the grooves (and not knowing whether Long Branch cut rifled, button rifled, or broached) I don't know which rifling mechanism put them there. But they're still there after all that lapping. And probably 2000+ FMJ rounds and at least as many cast bullets.
I also have my Grandpop's 1895 Winchester chambered in 30 U.S. aka 30/40 Krag, handed down to me after my Dad's death. He bought that rifle before the turn of the century and no doubt shot a lot of corrosive ammo through it before things changed. I don't know what the rifle barrel was when he bought it, but slugging the bore now shows that it has dimensions that would better fit a .303 British.
And when you look at the bore with a borescope, it takes a while to decide whether there's anything left for rifling of any depth or not. Which might be why the best it can do with any kind of 30/40 jacketed ammunition could be charitably called an 8" group at a hundred yards, even with the aperture sight on the rear and that long, long sight radius. Properly fitted cast bullets from my Accurate moulds for that Long Branch - and after much trial and error, it's just a tad over 2" groups at a 100 yards. And that's with a bore that looks like the inside of a sewer pipe through a borescope.
I am deeply tempted to take some medium steel wool to the bore for some lapping every time I look in that bore through a borescope. But then I remind myself this rifle's main purpose is hunting elk and deer in the thicker stuff as my Grandpop did, and I already have perfectly good 2" hunting loads with cast bullets in this rifle. And remembering the Long Branch, there is absolutely no guarantee that any kind of lapping will make any kind of significant difference.
Just some observations. I've never tried the NECO/firelapping thing. The commercial kits didn't exist when I first lapped, and my lapping days are probably behind me now. I do know that it takes a hell of a lot of lapping, even when doing it with medium steel wool, to remove all tool marks in a barrel. And just because a barrel looks terrible through a bore scope, doesn't necessarily mean it won't shoot well.
So I guess what I'm trying to say is that finding a definitive answer concerning lapping and/or bore condition is kind of like trying to nail Jello to a wall.