Well, I’m still tuning up my H&R in .38-55, using a globe front sight with interchangeable inserts, and a marbles tang sight with interchangeable eyecups, adjustable in elevation and windage.  I’m near sighted.  For my shooting glasses, I’m using clear glasses that are weak for the targets at 100yds, but have the front sight in perfect focus.  For the tang sight I’m choosing the smallest aperture that allows me to see the target clearly, trying to make use of the pin hole camera effect to get great depth of field so the target appears in focus also. 

  For targets I’m using the largest black circles I can find.

  For the front sight inserts I like the different sized rings supported by thin horizontal and vertical bars.  The Horizontal and vertical bars make it easy to be sure that you are holding the rifle vertically.  The ring’s inner diameter is just slightly larger that the apparent diameter of the target.  Lining up on target gives you a thin circle of light between a black ring and a black dot. 

  Any movement off target shows up immediately as a crescent of light pointing the way to move to get back on target. Try different inserts with different sized holes.  A very narrow circle of light is hard to hold still, good for bench rest shooting.  A slightly larger hole in the insert will relax the contrast and might be a better choice for offhand. 

What inside ring diameter would be just a little bigger that apparent diameter of a 10 inch bull’s eye at 100yds?

Let’s run through the numbers. 

What decimal inch size hole / sigh radius inches = target diameter inches / target distance inches x/30 = 10/3600 = .08333 = apparent size of target diameter

Front sight aperture hole would need to be roughly half way between 1/8 and 1/16 of an inch in diameter.

 A check of modern targets and the distances at which they are shot against the size of apertures available might shed some light on which aperture was thought best for each target diameter / distance.  It is as easy to measure the aperture it is to measure a drill bit that just fits it. 

The conventional shotgun cartridge is designed for black powder. It was invented around 1870 when black powder was the only kind of gunpowder that anyone had ever heard of. They didn't even call it "black" powder. It works just fine with black powder, and can still be loaded with black powder today. Here is how. 
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This is not a complete treatise on reloading and is only about the unique problems caused by substituting black powder for smokeless powder in shotgun shells. The reader is assumed to be familiar with the tools and procedures for loading modern shells. 
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The main problems with black powder relative to smokeless powder are: 
  • Black powder takes up more room in the shell than smokeless powder. 
  • Old guns often have short chambers; the shells must be cut short to match. 
  • Plastic melts. 
  • Smoke and Dirt. 
  • Rust. 
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Black Powder is a simple mixture of potassium nitrate, sulphur, and charcoal. The proper granulation is named 'ffg'. It is also called '2F'. Anything finer is dangerous because it burns too quick and the pressure climbs too high. Courser granulation burns too slow. Buy black powder from muzzle loader dealers. Most general gun shops do not stock it anymore; keep shopping. Some brands of powder will produce little red-orange beads of molten sulphur that melt small holes clear through a plastic case. Try another. 
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Modern factory loads are often labeled in 'drams equivalent'. That means that whatever charge of whatever kind of smokeless powder is in there is supposed to produce the same shot velocity as with so many drams of black powder. The peak gas pressure is not the same and nether is the pressure curve.  The avoirdupois dram is the unit of weight used to measure black powder. There are 256 drams in a pound avoirdupois, 16 drams in an ounce, and 7000 grains in a pound. Take care not to confuse 'dram' with 'drachm' or 'dram apothecaries' which are not the same things at all.

1 ounce = 16 drams = 437.5 grains.
1 dram = 27.34375 grains = 1.772 grams.
1 gram = 15.432 grains = 0.564 drams.
 

Quite a variation. That is why you measure powder by weight and not by volume. The average seems high to me; 235 is the value that I would likely use when making a powder measure. 
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Chamber Length. Many old guns have short chambers. You will have to trim the modern shells to fit. If a long shell is fired in a short chamber there will not be room for the mouth of the case to open and the pressure will increase dangerously. Shells are measured while open; not when crimped. Old barrels are seldom marked with the chamber length; you must measure them to find out. The modern 2 ¾ inch shells were introduced in the 1920's along with the star crimp. 
 

A simple chamber length gauge may also be made from a bit of heavy sheet metal. This one is double ended; the pictured side is 12 gauge and the other side is for 16 gauge. 

Also note that old shotguns often have chambers that are shaped differently from modern ones. Here is a Cerrosafe chamber cast of a Parker gun made in 1890. There is hardly any forcing cone; the chamber simply ends with an abrupt step at 2 5/8 inch just like a rifle chamber. It's diameter at the case mouth is right at the modern minimum of 0.798. This is meant for a brass shell. 

Cerrosafe is a bismuth based alloy that melts at about 180°F. Just hold the barrel horizontal, clap a bit of wood at the breech with your fingers to make a dam, and pour it right in. Avoid the extractor. This makes a nice cast image of one side of the chamber and you can see the reamer marks and rust pits and everything. Get it at www.brownells.com. 
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Proof Marks. Most of the large European countries have proof laws.  New or repaired guns must be submitted to the official proof house where they are fired with very heavy charges. Those that do not happen blow up are given Proof Marks. They can tell you all sorts of things about where and when the gun was made and what sort of load it was designed for and sometimes even the original bore diameter and chamber length. The decipherment of proof marks is a very large subject and if your gun has any you really should get the book The Standard Dictionary of Proof Marks by Wirnsberger and read up on them.

...

The Birmingham Gun Barrel Proof House is still in operation and on The Web.

The United States does not have a proof law; each factory has it's own private marks but they are boring and no where near as elaborate as the European ones.

Plastic Shells. Plastic shells get sort of melted on the inside when fired with black powder and will not take a crimp after about 2 or 3 firings. Just accept it and throw them away. Paper shells work fine; they were originally designed for black powder. 
I normally use plastic. It is easier to obtain and is easier to trim and crimp. 
 

Plastic Shell Trimming trick. Make a brass disk 0.25 thick and 0.72 inch diameter (for 12 Gauge). Drill and tap it with a ¼ x 20 screw thread. The soft metal is so it does not scratch up your knife.   Ram it down on top of the over shot wad in an untrimmed plastic shell. Hold the assembled shell tight in one hand and with your other hand run a sharp pen knife around the case flush with the disk. The blade lies flat on the disk and your thumb slides on the outside of the case to guide it. It is sort of like peeling an orange. Check the shell length to make sure it is correct for the chamber.

Solid Brass Shells. These used to be popular as they last dozens of firings and are easy to load with simple tools. I have not actually loaded any brass shells but here is what I have read:

In Shotgun Digest by Robert Stack ISBN 0-695-80497-9 there is a so-so 10 page chapter (mostly pictures) entitled "Brass Ain't Crass".

In The American Rifleman October 1964 page 32 there is a much better 3 page article "Loading Brass Shotgun Shells". It describes the various brass shells that were available brand new in 1964 and the various primers to fit them. Tells how to seal the over shot wad with water glass (sodium silicate). This stuff is still sold in big drugstores as an egg preservative. Mix with 2 parts water and put it in with an eyedropper and let it dry. Alcan used to make special oversize wads just for brass cases; they don't any more, but Circle Fly does. Black power is mentioned like it is normal. 
Here is that article as three .gif files:
bpsg-p1.gif  Page 1
bpsg-p2.gif  Page 2
bpsg-p3.gif  Page 3

If your web browser can't print these files and get them to fit on paper then you might try saving them to disk and printing them with some other program that can.

And here is another article from The Dope Bag of November 1931.
Here is a picture of a 12 ga Brass shell. The wall thickness is 0.011 - 0.012 inches and the length is 2.53 inches. Takes a Small Pistol primer. It has a balloon-head and there is no base wad so there is plenty of room inside. 
You can make brass shells for the .410 shotgun by fire forming .303 British, .444 Marlin, or 9.3x74R rifle cases. 
Low base wad. The 'base wad' is located down inside the shell around the primer. It has nothing at all to do with the amount of brass on the outside. A low base wad is desirable because it leaves more room for the black powder. 
Primers. Black powder is extremely easy to ignite. Any old primer that fits the pocket will do. There will be no difficulties with poor ignition. 
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Wads. The modern plastic wads take up too much space in the shell so you will probably have to use the old fiber wads. Fiber wads can still be bought from dealers in muzzle loading supplies. High quality fiber wads can sometimes be obtained from Trap shooters who have long since converted to plastic but who could not bring themselves to throw them away. 

Wad Pressure. Black Powder should be packed pretty tight but that is not critical. Even if left slightly loose it will not give 'squib loads' like smokeless sometimes will. 
 

Roll Crimp. If there is sufficient space remaining in the case after the shot is added then just go ahead and use the modern star crimp. If the shot leaks out through a star crimp a nice trick is to stick a ¾ inch square of paper over the shot and then crimp over it. That will seal the case without spoiling the pattern.  If the shell is too short for a star crimp you will have to use the old fashioned roll crimp. Get yourself an old roll crimp tool. This is a typically a little machine that clamps on the edge of a workbench. There is a chamber to hold the shell, a lever to press it in, and a crank to turn.  Crimpers can often be found in antique shops; they are about as common as parlor stereoscopes. The price seems to depend on whether or not the shop owner can figure out what the hell it is; an item labeled "Quaint Olde Tyme Vintage Tool" always seems to cost more than a "Shell Crimper". They can also be found on eBay. There is a separate size tool for each gauge.

Old Ideal shell crimper

The head of the Star Crimper made by Lyman can be chucked in a drill press.  A roll crimp requires a special over shot wad. It is made of thin but strong cardboard; strong enough to stand recoil in a double gun but thin enough that it will not spoil the pattern. This pattern spoiling is the main reason why modern shells use the star crimp. Stick the card wad over the shot and get it nice and level. There should be about 1/4 or 5/16 inches remaining in the case. Put the shell in the crimping tool and press it in with the lever. Turn the crank until a nice crimp is formed.  If you get a few bad crimps just load them in the right hand barrel and shoot them first. Save the good ones for the second shot. You don't want the crimp opening up under recoil and leaking shot down the bore.

Cases that have never been star crimped are easier to roll crimp. Shells that once held rifled slugs work nice. Star crimped shells that have been shortened work OK. Star crimped magnum shells work fine after shortening because all of the original crimp is removed.  You can write your load data right on the over shot wad. I use a black "china marker". Consider having a special rubber stamp made. The finished shell may be rendered watertight by painting the wad and crimp with shellac, or hot wax, or cheap nail polish, or water glass, or whatever.

Wad Labels

Recoil. Recoil is somewhat greater with black powder than with an otherwise identical modern load. It has to do with the greater mass of gas and dirt being ejected from the muzzle. Initial acceleration is sharper due to the large initial surface area of the grains which decrease as they burn. Fiber wads are heavier than plastic too. Get used to it. 

Modern smokeless powder is advertised as 'progressive burning' giving 'lower recoil' and 'low flame temperature'; black powder has none of these modern advantages. 

Noise. Black powder is a lot louder than smokeless. Get used to it.
It is good for scaring people in the trap squad. The first time I tried it it even startled the guy in the concrete trap house.

Dirt. Black powder burns dirty. Learn to like it. Expect the bore to be lined with a thick coat of crud. 
The chamber however will remain clean, there will be no gas leaks at the breech, and there will be no jamming problems even after hundreds of shots without cleaning. 
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Black powder substitutes. I normally use only real black powder, but for extra money there are a few modern powders to consider: 

Pyrodex®. Substitute it for black powder by volume, not weight. Use the 'RS' grade. Pyrodex® is very hard to ignite. It must be packed very hard or else it will not burn correctly. Consider adding a little real black powder next to the primer to help it catch fire. It is very light and spongy stuff and can be squeezed down to about 2/3ds of it's original volume. I guess it it is made that way to mimic black powder's bulk. The residue is said to be even more corrosive than black powder's. 

Black Canyontm powder was crap. They don't make it anymore. 

Clean Shottm powder is OK stuff. It behaves about like black powder. Contains no sulphur so there is less stink and no holes burned in plastic shells. Dissolves instantly in water. Less fouling, and what there is is not so hard and crusty. Substitute by volume not weight. It must be packed tight to burn well but not so very tight as Pyrodex®. It's residue is said to be more corrosive though so take extra care in cleaning. 

Clear Shot. Made by GOEX. It's strange looking stuff. I haven't tried it yet. 

American Pioneertm Powder. I haven't tried it yet. 
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Modern Factory Load. Here is a picture of the insides of a modern 12 gauge Gamebore brand cartridge. The case is red paper with a thin plastic base wad, 65mm (2½ inches) long and roll crimped. The base is plated steel. Overshot wad is cork with plastic film on both sides; it should disintegrate on firing. Shot is 28g (1 ounce) of #6. Fiber wad also has plastic film on both ends. Powder weighs only 2½ drams and looks like FFFg. A very light load. 
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Clean Up. The fouling from black powder is corrosive. It must be cleaned out very soon after shooting or the bore will rust. 
The fouling is water soluble; it will not dissolve in oil. Only water will work. You can buy various special cleaning products for black powder if you really want to but their active ingredient is water. 
 

A typical late 19th century breech loading shotgun will have a removable barrel containing only one movable part, the extractor. Water will not harm this thing; it was designed to be cleaned in water every day. If it is all rusty inside that is a sure sign that one of it's former owners did not use enough water.  Make sure that the gun is unloaded. Remove the forend. Remove the barrels. Wipe off the breech face and oil it; You are done with that part. Take the barrel outside. Squirt water down the bores with a garden hose to remove half of the dirt. Stick the muzzle in a wooden (or plastic) bucket of cold water and run a wire brush through a few dozen times.

Change the water and repeat. It is now clean.  Note that the cleaning part is done with cold water. Now get some very hot water. Pour a gallon or so down the bores. The point of this is to heat the metal so that water will evaporate fast and dry before rusting. If you don't feel like boiling water on the stove then just take it in the bathroom and tie a sock around the breech and hang it under the hot shower for half a minute like I do. Dry it inside and out while hot, let it cool. Oil the bores, extractor, and lump. Oiling the outside is optional; you might want to build up a nice patina. Reassemble. Done. Easy.

I have never had any rust problems with guns cleaned this way; but then I live in California and this severely limits my experience with corrosion. 
Without the new-fangled plastic shot cup the shot will rub on the bore and lead it. Turpentine helps to clean it out. 

There is more info on Tom Bullock's Web site.