Reloading for a shotgun is not particularly challenging. People who know me would tell you that if I can do it, then it doesn't take a lot of mechanical skill. Some go so far as to tell me that I'm dangerous with anything more complex than a screwdriver in my hands.
That being said, let me go back to your question about determining choke. It is actually very easy, but you do need the right tool to do it right. When I first started out, I had a drop in bore gauge. Drop it in from the muzzle and you get a choke reading. But that only works if you're starting out with a shotgun that has a standard bore diameter. The tool that does the job accurately is a bore and choke gauge. Not all that complex, but far more accurate than the drop in gauge I used initially. You will find plenty of shotguns--especially older ones-- with a bore that doesn't have the standard diameter of .729-.730 (if it's a 12ga.) The bore and choke gauge has a dial on one end attached to a rod which goes into the bore from the muzzle. The rod has a head on it which measures bore diameter which you read on the gauge. Then you simply slide the tool out of the barrel, and as you get closer to the muzzle, the reading will begin to change, showing constriction. Which we call choke. You can find all sorts of charts, mostly fairly accurate, which tell you how much constriction equates to what choke on a gun of the gauge in question. The smaller the bore, the less constriction is required to produce the same degree of choke. For example, while anything in the 30 thousandths range would be full choke in as 12 gauge, you only need about 25 thousandths constriction to give you full choke in a 20 gauge. Most of us do have to refer to choke charts in order to remember what is modified on a 16ga, for example, compared to a 12 or a 20. Just remember that choke is the difference between bore diameter before constriction begins to take place and what your gauge reads as you pull it out of the muzzle.
