josquin said:
Thanks for this. I have used pure lead (fishing weights) for slugging the bore and I know how hard those Bullet Barn bullets are (BHN 22, as I recall. They must quench them as well).
Don't know anything about them. I would assumed they would have looked at the economics of purchasing WW metal in bulk versus a designated alloy. I do know that if I were a commercial casting firm, I would heat treat instead of drop quench - a great deal more control over final BHN, and more uniform hardness from bullet to bullet. Also allows to specifically choose a BHN for your finished product in an attempt to match expected operating pressures of the loads your customers will probably be using. And easier to make your suggested loads (if you provide them) have a greater chance of success in customer's firearms.
Interesting about the grits. I would naturally have assumed "the finer the better" and was a bit surprised to see that the Wheeler kit I have contained 220, 320 and 600 and nothing finer. Maybe I'll have another go at the Martini!
However, I don't know that it's likely to get better than this:
Steve Jennings knew more about barrels and bullets than I'm ever likely to learn in several lifetimes. And he had (or had access to) a very expensive borescope which I don't have. I've never had any interest in following his lead and polishing to a 1000 grit finish or whatever to confirm what he told me was correct. You will find once you get into the 400 or so grit finishes, you have a very fine finish indeed.
From the picture you attached, I would say you can improve that bore quite a bit with hand lapping. The beauty of the cast bullet is you can adjust bullet diameters to fit just about any bore/groove diameters you want. You can either lap the mould out as well so it throws the diameters you want, or get NEI or Mountain Moulds to make something that drops bullets at your exact specifications.
For example, NEI makes a 200 grain .316" bullet mould that would probably be the cat's ass (or, at least as likely to bring success as anything else). You could shoot it as is, size down a thou or two - or simply have NEI cut it a thou or two bigger in size. if more is required. I expect if you were to get that barrel down to somewhere in the .316 - .318" diameter range, most of the pitting and corrosion is going to disappear.
I've read that getting the lap started is the dangerous part as it is easy to damage the crown and beginning of the bore as the lap enters. I think if I had the room I'd almost be inclined to make some sort of a jig or rest to lay the lapping rod on to minimize starting it crooked.
Hmmm... well, you're working from the chamber end, so damaging the crown should be difficult, except for ham-fisted bubba's. You pour the lap, push it out slightly to cut the "mushroom" off the end so the lap will completely enter the barrel, coat the lap with Clover, and then start working from the muzzle to the ball seat. You mark your cleaning rod - so the lap should never completely exit the muzzle or enter the throat anyways. If it does, you simply carefully realign the lap with the lands and grooves (it's the only way it will go back in anyways), and back into the barrel it goes. The odd tap or two with a brass rod from the front to bump it up in diameter as you progress, and that's about all there is to it.
That lap is dead soft, and I can't see how one or two instances of having to restart it into the barrel is going to do a significant amount of cutting - it takes a LOT of work to make progress as you lap the imperfections out of a barrel. Unless you're using a really aggressive grit, of course.
For the faint of heart (not a bad thing with a treasured firearm), practice lapping a junked barrel. You'll get a feel for the process and you won't have to go to the gym for your workout that day... As you mentioned the original barrel is due for replacing, have a go at keeping it all original by lapping that barrel out to a decent finish. You'll be no worse off than you are now, and you may salvage an original barrel that right now is a write off.
You mentioned you couldn't remember what the bore slugged at. As that appears to be a five groove barrel, how did you get your measurements?