Sounds like someone got their money's worth out of the old Hollenbeck based on your description; so sometimes it's just best to preserve a battered old relic as it is and hang it on the wall. But if you do decide to do something with this piece, you need to know some basic things about SAC guns. First of all, the fact that the name on the gun is HOLLENBECK means that the sn# is most likely below 5K. It also means that you have what is known as a FIRST MODEL frame, and that the forend hanger is composed of a flat V spring device as opposed to the later J spring
type fastener. First model frames were in production till about serial number 24,000. At that point the second model frame was introduced for about two years, then the frame modified again with a change in the cocking mechanism to create a 3rd model frame. All this means that barrels from these three models are not interchangeable. Since you're a machinist, and assuming you realty wanted to take on such a project(and would likely never find a set of 10 or 16 gauge barrels), you could look for a set of 12-bore First Model barrels and fit those to your frame. Doing so would require relocating firing pin holes in the breech face, modifying hammer strikes; then filing breech balls to the new correct profile. If you did this you'd also be wise to find a barrel set utilizing the J spring fastener and J spring fore iron, as Hollenbeck's V spring device does not hold the iron as secure as the J spring so that the fore arms on these early guns will often detach during recoil. Again, best of luck.
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