Lexan sheet is a shotgun patterner's dream material. Years ago, when working on the chokes of a doubel AYA 10 bore, I made a 1/10" thick, 30" circle of lexan for use in patterning my guns. Further experimentation led to cutting a 20" circle out of the middle of this sheet, just to get the concentration percentage differences for the inner and outer circles which is valuble in determining a load's useful range. Nicks in it's edge allowed a felt pen to mark the quadrants for later scribing straight lines out from centre with the felt pen. This made counting holes much easier and for seeing 'holes' if they developed inside the pattern.
Patterning got to be quite time consuming, and if working on a cylinder bore or at close range today, I'd probably forego the 30" and merely stick with the 20" pattern sheet. After the 10 bore, came a couple 12,s and a pair of 20's, all of which benefited a lot from load development and alteration of wad columns.
Our BP shotguns are no different in regards to developing patterns, except I've found working with BP loads to be somewhat easier than with modern stuff. The force with which the load is seated in the breech, is quite important in getting repeatable results in patterns, btw - same pressure each and every time and about 40 to 50 pounds of force will suffice. However, if one load has 10 pounds and one 50, the pattern will reflect this difference quite noticably, giving little consistancy in the delivered pattern, just the same as if one ball is seated with little pressure and more for the next. They don't hit the same place on a 50 yard target. No pressure in one shot load can produce huge holes in a load that produces normally excellent patterns.
Patterning and thus load development can take a weak load putting a mere 40% to 50% of the shot inside that 30" circle (cylinder - no choke) to landing a nice, slightly centre concentrated pattern in the 70 to 80% range. Given the proper sized shot, this can turn a 25 yard gun into a 35 yard gun for upland or ducks over decoys, or even allow good wing shooting to 40 with tighter patterns yet. Choked barrels usually improve this range in 5 yard increments, ie: ImpCyl, Modified, IMP Mod, then full.
Black powder, although thought by many to be an archaic propellent form, really isn't when it comes to shot loads. BP guns usually pattern better than similar choked modern guns which are using smokeless powder. BP shotguns are also more load friendly in that they are easier to work up loads for compared to a modern gun as well.