Hi, Southpaw:
5 Shot is right, your barrel may larger than the commercial jacketed bullets. I've lapped two barrels with Marshall's supplies. One was a new rifle that is now cured of a horrible case of copper fouling. The other is an oldie that needs another 20 or 40 rounds, but is better than it was. I was not aware of the Neco process (using jacketed bullets) when I did the new rifle about 6 years ago.
You could check the bullets you've got on hand by firing a light load into wetpack or water and seeing if it's getting to the bottom of the grooves. It's easy to see if it's still shiny between the land marks. It's probably more accurate than trying to measure a slug from a 5 groove barrel (S&W or Lee-Enfield) without the special V block normally used.
I saw a picture of someone, probably Seyfried or Venturino, slugging a barrel from the rear. He put the centre of a long steel bar against the slug, held one end by hand and hammered the other end. More likely, he had the slug as close to the hammer end as possible. Anyone remember that article?
I don't blame you for being cautious and I don't doubt that some barrels have been damaged by firelapping. Whether it's operation error or a cranky barrel is a good question. Precision Shooting Magazine gave the Neco system some pretty favourable coverage a while back and the benchrest crowd would have panned it pretty quick if it didn't work. (I dropped my subscription when the international mailing rate went through the roof.) However, these guys aren't shooting revolvers. The top grade benchrest barrels are better left alone but some production varmint barrels need help. A rifle barrel can't have a thread choke because the chamber extends past the action and most varmint barrels don't have dovetail cuts. So these barrels are pretty uniform in diameter but a bit rough and the Neco system must do a good job of slicking them up. Also the jacketed bullet spring back that Marshall refers to isn't a problem if the barrel has a constant diameter.
If we have a serious constriction, we don't want any spring back. The other problem I see is that we may not get the constriction cut out before we move to the finer grits, which won't cut it out. So we should slug after every 20 shots and see what's happening. I've read comments from two of the top barrel makers, Gale McMillan and Dan Lilja, who strongly object to the extermely fine 800 and 1200 grits.
There's another problem with a minimum sized bullet. Suppose we have an .001 constriction. Cut it out and we're trying to lap with an undersized bullet, and just washing out the lands.
Hand lapping is fine for people like Mr. Gates who have their technique down pat. I think firelapping is a better choice if we don't have an old hand looking over our shoulder on our first attempt.
New production (not custom) chrome-moly barrels are blued on the inside. I've only seen one article on this, but the author claimed they won't shoot until the blue is out. He said you can shoot it out with 200 rounds or scrub it out with JB (best) or use blue remover (tricky). He used wood plugs in the Thirties, but now uses teflon washers to seal the bore, and claims they break in a lot faster.
Anyhow, I'd run at least 200 jacketed bullet through it before lapping unless it's fouling something terrible. It may not need lapping at all.
Bye
Jack