Easiest way I am aware of is to use brass shells with large pistol primers with 6-10 grains of unique or other black powders/replacements, 1/2" fiber wad on the powder with a 1/32" overshot wad above the shot and then epoxy the edge of the outer wads and dry. If you are doing buckshot, it is typically smart to evenly stack them in layers and use some shot buffer in a thin layer in between the layers of shot to keep them from deforming so bad. Just carefully arranging them so they interlock helps also and acts like a spring on firing. Here's a link to one place to get the 16 ga. brass shells:
There are components available from Ballistic Products Inc. as well as a wealth of data. You can buy brass hulls for the 16 ga. If you do, be aware that you cannot use modern plastic wads, they are too small. You will need to buy traditional components and they should be probably 14 ga. in order to fit a brass 16 ga. hull. Another source is Circle Fly wads.
The best hulls to use, IMHO, are paper hulls but they can be very hard to find, especially in 16 ga.
If you load plastic hulls, and you can, they will be good for one firing only.
Powders - BP. Use real BP if you can get it. Use dram loading (a dram is about 27 grains). A 16 ga. 2 3/4 dram load will have about 67 grains of BP (or the same volume of a substitute like Pyrodex) and 1 1/8 to 1 1/4 oz. of shot.
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