Wingbone,
I don't doubt the 'smith did a workmanlike job on the firing pin. Without more details being posted, I do question what you say he said, or your understanding of his advice to you. Please, I'm not trying to be rude, but I do want you to enjoy your old Forehand- safely.
1. Nineteenth century black powder shotguns were designed for the peak pressure developed by black powder. This pressure is, in normal period loads, (very) roughly half of what some modern low brass shotshells loaded with smokeless powder routinely produce.
2. Perhaps the 'smith was loading his own shells, indeed, there are some few smokeless powders which, properly loaded, can be used to produce what are, by modern standards, very low pressures, and to function with acceptable, if not great efficiency. Nevertheless, the shape of the time/pressure curve differs somewhat between smokeless and black powder. The 19th century designers weren't fools, they developed a gun safe and suitable for black powder ammunition.
But, when smokeless powders were introduced it was generally found necessary by the arms manufacturers to build guns which were capable of operating at considerably higher pressure over the "life" of the gun:
this was done for the safety of shooters.
3. Therefore, I'd strongly recommend that you use
only black powder shotshells in this gun.
4. I'd also recommend that you have the gun examined by a specialist gunsmith, someone who knows antique arms. One way or another, it's imperative that you thoroughly understand the capabilities and limitations of your gun and the proper ammunition for use in your gun before you use it.
5. If you have "twist," "Damascus", or similar barrels I'd read up on the topic before firing the gun. Internal corrosion is not uncommon. That the gun was recently "proofed" may, or
may not prove its' safety: the causes of failure of old shotgun barrels are not simple; it's well established that some guns in apparently good condition have burst when the proper ammunition was used. The cost of testing using modern procedures can exceed the value of the gun in some instances. Still, the opinion of a well established specialist 'smith who has examined many such guns is well worth obtaining.
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Late nineteenth century black powder shotshells almost invariably propelled what by today's standards is a relatively light load of shot: in 12 gauge, 1¼ oz was about as heavy a load as was commonly employed, 1 1/8 oz and 1 oz loads were probably more frequently used. Many guns made overseas were subject to proof by governing bodies, the service load is often stamped on the flat of one of the barrels. Here in the U.S. this information is not usually to be found stamped on the gun, loads used here were sometimes a bit heavier than those used in Europe (due to differing conditions and hunting methods), but a load like a 1 5/8 oz "baby magnum" can't be fit into a standard 2 1/2 or 2 5/8" black powder case- at least with a servicable wad column. That's right- 2 1/2 or 2 5/8" case. These were the standard case lengths here in the U.S. in the late 19th century.
You
need to know your chamber length, if you don't
know it positively. Yes, it's possible that your Forehand could have had its' chambers originally cut to 2 3/4", or had its chambers lengthened long ago. Lengthening chambers is sometimes safe, and sometimes not safe- it depends on the individual gun. Yet another reason to seek the opinion of a
qualified gunsmith. Yes, 2 3/4" shotshells can more often than not be fit into a shorter chamber, but to fire 2 3/4" shells in a shorter chamber will sometimes produce
very high pressure.
BTW, correctly made firing pins for smokeless arms are smaller than firing pins that are made for black powder (only) arms. In a nutshell, this is because developed peak chamber pressure is higher with standard smokeless cartridges, yet another reason to aviod shooting smokeless powder in an antique arm.
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I've never purchased black powder shotshells. I have, however, loaded thousands of them, and used them in antique arms, both in hunting and in the clay bird games. Therefore, I can't endorse the following suppliers of black powder shotshells. But I will offer comments based on the online descriptions of the offerings from the following vendors;
Republic Metallic Cartridge offers true black powder shotshells. These may be the best you can purchase. They use a traditional wad column ("biodegradable wads")- not biodegradable plastic wads. All plastic wads leave bore fouling to a greater or lesser extent when used with black powder so this is a step in the right direction. Personally, I prefer paper hulls (or brass) to plastic for a number of reasons, but you may find these very satisfactory. Unfortunately, these are available in 2 3/4", 3 dram, 1 1/4 oz of #7 1/2 shot only.
Ten-X, a manufacturer of ammunition for Cowboy Action shooters produces a black powder substitute shotshell loaded with 1 1/8 oz of shot, shot size not stated, but probably a relatively small size. Again, it's available in 2 3/4" only. You might write or call for further information.
There are a couple of other outfits making specialty loads- before you go there I'd recommend that you get standard loads working well first.
regards,
Bob