From census records, I have been able to determine that Abraham and Adam moved from Pennsylvania to Germantown, Indiana, west of Richmond before 1850. The 1860 census shows Adam living in Tipton, Indiana. (I have not found any information in the 1860 census about Abraham, but he was buried in Arcadia, Indiana, which is near Tipton, so it appears that he moved close by to Adam before he died.). Adam is buried in a cemetery in Tipton.
You can't see it from my bad pictures, but the lock is marked "Underwood Lafayette, Ind". Underwood apparently was a hardware dealer during this time. This helped me establish that the rifle was probably built in Indiana. It also helps explain how my great great grandfather came to possess this rifle, since they both lived in the same county. What's not clear, is who actually made this rifle. My money is still on Adam, since Abraham would have been 66 years old at the time of the 1850 census.
Thanks to Art DeCamp for helping me with the information on the Angstadt move from Pennsylvania to Indiana.