I've firelapped several Garand barrels, checking them with a throat wear gauge before and after. The military throat wear gauges allow a throat to be pushed back ten thousandths (that is, a hundredth of an inch, not some tenths of a thousandth) before being rejected as worn out. Match shooters like to limit it to five thousandths. I found firelapping pushed them back one thousandth, and that is after using twice the number of lapping loads normally recommended because the constrictions below the lower band perch on the barrels were so bad.
Additionally, you should know that throat wear from firing is different from what firelapping does. Erosion by normal shooting is the result of heat stress cracking of the throat surface until it forms an alligator skin pattern and chips off. This causes an uneven, rough surface. Firelapping, on the other hand, produces a smooth, even surface that is free of tool marks and tends to produce less metal fouling.
That difference in resulting texture is important. David Tubb's Final Finish bore polishing system (not a firelapping system, as it abrades the bore evenly all over to resurface it, and is not aimed at removing constrictions by abrading them more than the wide spots, as firelapping does). What he has found is that he can sometimes double the life of a barrel by resurfacing a shot-out throat with his Final Finish system. It is obviously not putting any lost metal back, and the resurfaced throat has a more tapered throat than the original chamber started with. But some experimenters, like Harold Vaughn, think that steeper taper actually makes an accuracy improvement by centering bullets better. In Tubb's experience, getting an eroded throat smoothed revives the rifle's accuracy despite the missing metal.
From the above, I think when you compare a throat set back a thousandth by firelapping to a throat eroded that much by firing, you are comparing apples to oranges. Having firelapped a number of barrels using the NECO system, I find far less reason for concern about the throat than you express. But I also understand if you don't find this persuasive. I approached firelapping with a lot of trepidation the first time I did it, too.
Traditionally, hand lapping uses a cast lap. This is made by plugging the bore an inch or so below the muzzle with a cotton rag (you can't buy asbestos paper anymore) and pouring the lap alloy in. The lap is gently tapped back out with a brass rod in the bore. It needs to be marked so it can be put back in with the same orientation with respect to the muzzle. Grooves are then carved into it with a knife that are kind of like cast bullet lube grooves, and are loaded with abrasive. The lap has to be cast onto a threaded rod or a screw or a much-undersized brass jag, so it can be engaged by a rod for working back and forth in the bore.
Personally, I've used a bullet mold for a long multiple lube-groove bullet to make hand lap. I cast the bullet from pure lead or something very close to it. I cast with the sprue plate open so I could hold an 8-32 screw centered upside down in the mold cavity while pouring the lead in around it, then carved the overflow away with a knife afterward. Before casting, I cut the head off the screw and hammered a flat into it for grip by the lead and put that end in the mold for casting. That lets the cast metal grip it without a screw head interfering with the pour or, worse, setting off center enough to rub the bore. The tip of the screw stuck up out of the lap for a cleaning rod. I tapped the soft lead bullet into he muzzle with a little light oil on both it an in the bore before applying abrasive to it. That got me a matching fit. I then backed it out far enough for the lube grooves to be loaded with abrasive.
The lapping itself is just a matter of working the abrasive-loaded lap back and forth with the rod. A muzzle guide for the rod is a good idea as it will otherwise abrade the lands of the rifling near the muzzle, what with abrasive traces being present. The custom barrel makers normally lap the barrel blank before it is trimmed to length, so that rubbing isn't an issue for them. I turned the muzzle guide from Delryn on my lathe. There are probably other things you can think to do.
The lead lap is narrowed passing through constrictions, and when you feel it stop rubbing much there, you need to run a brass rod into the bore from the other side and tap it with a hammer to bump the lap back up. You keep repeating and periodically refresh the abrasive until you feel the lap rub evenly in the whole bore. At that point constrictions are gone. Whether or not the whole bore surface is satisfactory is something you need a bore scope to see for sure. You should get a clue by looking at what you have near the muzzle if you don't have a bore scope, but running slugs through to feel the evenness will get you more information.
If you use an alloy that's harder than lead you will do less bumping, but it is going to widen the wide spots a little, too, unless it is cast. A cast version will shrink a little and bumping it up to cut enough is then harder than bumping up softer lead, but you do a lot less bumping in the end. It seems to me the lead bullet lapping I did took forever. I stopped more than once during the process, so I don't really know how much time it took. A harder alloy would have gone a lot faster, but I'd have had less control. There are always tradeoffs you have to explore as you figure out how you can best do the job. That's the skill aspect of it, and I'll admit that one experience is part of what interested me in firelapping.
By the way, Veral Smith says his measurements show that constrictions under half a thousandth don't tend to cause much trouble. You may want to slug your bore and find the size of that constriction before you make a lot of extra work for yourself.
