That is an interesting old gun. The front sight installation appears to be a professional job, and the ramrod pipe under the barrel supports this. I know a lot of M1842 muskets were put back into service during the War Between the States. After the war, the United States Army was downsized and thousands of military arms were sold off to the public. A great many were "sporterized" by being cut down and half-stocked, and rifle muskets were reamed smooth. The enigma here is the stock being cut off flush with the barrel band. Most of the professionally modified guns had a few inches of wood protruding beyond that lower band after they were half stocked. Also, removal of the buttplate is unusual for a professionally modified gun. The original buttplate on this musket was flat, and pretty uncomfortable to shoot. The slightly rounded profile of the butt on your musket actually looks more comfortable, without the plate.
As for what to do with it, I have some opinions. I don't think it would be cost effective to try to restore it. There are still a number of more or less intact "forty-twos" in circulation, frequently selling for about the same price as a new modern reproduction. If the condition of the bore matches the outside of the barrel, it may be pretty rough. Dan Whitacre can make a new smoothbore barrel for you for $525, or a rifled one for $550 (see
Whitacre's Machine Shop). Bob Hoyt could probably smooth the bore of the existing barrel, or line it, for a bit less than that, but I just spoke with Mr. Hoyt a few days ago and he is really trying to reduce his workload. He is still taking in a few simple jobs, but you would need to discuss this with him if you want to pursue it. In any event, a full restoration would likely cost you considerably more than buying a more intact and shootable original.
If it were me, I would want to shoot it. It sounds as if the original lock is still good, which is really not surprising. The original Springfield and Harpers Ferry locks were excellent. If the barrel is shootable or easily smoothed up, so much the better. I think I would try to graft on a piece of walnut to extend the forend ahead of the barrel band, just for appearance's sake.
When you take the breech out of the penetrating oil soak, you might try heating the bolster with a propane torch before attempting to remove the nipple. You don't need to get it red hot... just too hot to touch. If the nipple won't turn, let it cool, then heat it and try again.
I was thinking that, too. In fact here is an original cut-down M1842 with native associations, just to give an idea:
View attachment 255979
I like the simple, tasteful tack-work on this gun, which still has its buttplate. That's worth mentioning because lot of Indian guns had the buttplate removed. Some say this was so the metal could be used in making a hide scraper, others say Indians just didn't like metal buttplates against their bare skin on very cold or hot days. Whatever... I just know some guns with native provenance are missing their buttplates.
We are not supposed to post links to auction sites, but since this gun has already sold, I hope we are "legal" in posting a link. You can read more about this particular gun (and see a photo of the other side) right here, on the InValuable site:
Springfield Model 1842 Cut Down Musket
Best regards,
Notchy Bob