Howdy
I'm not sure where the Original Poster lives, but here in the USA it is typical to express the weight of shot in ounces, not in grams. Doing a quick conversion I see that 28 grams of shot is almost exactly 1 ounce. Which makes for a pretty light load.
Just as specifying the amount of powder and shot without specifying a wad is useless information for a Smokeless shotshell load, it is the same with Black Powder.
With the thin cross section of a shotgun barrel, it is essential that Smokeless components; hull, primer, powder, wad, and shot be specified precisely or bad things can happen.
Luckily, with a Black Powder load, the components are more forgiving. Still, you do not want powder and shot rattling around loose in a BP shell. You still have to specify a wad. Generally speaking, because the volume of Black Powder used in a shotshell is greater than the volume of Smokeless powder used in a shotshell, most modern plastic shotgun wads are too tall for a Black Powder load, and will not fit properly in the loaded hull.
Some shooters cut off standard plastic wads, but the most common modern plastic wad used by Black Powder shooters in CAS is the old Winchester Red Wad. It is a fairly short wad and fits well with the increased volume of powder usually found in a BP shotshell. Unfortunately, the old Winchester Red Wad has been out of production for some time.
Luckily, Claybuster makes a wad which is essentially the same as the old Winchester Red Wad, CB1138-12. You will see this wad about halfway down the page with this link. Notice that this wad specifies it is good for 1 1/8 ounces to 1 7/8 ounce loads. I have no idea if it will work with a 1 ounce load.
https://www.claybusterwads.com/index.php/winchester-style
Square Loads: The idea behind the old Square Load was to use the same Volume of shot as the Volume of powder. Generally speaking, putting more powder in by volume than shot tended to blow the pattern open. As has been stated, putting
less powder than shot, by volume, in the hull is not a problem.
Getting back to plastic wads for a moment, unfortunately the high temperature of Black Powder burning tends to cause the plastic to melt, much more so than a Smokeless load. This tends to leave melted plastic in the bore, which must be cleaned out when cleaning the shotgun. Generally speaking, very hot water along with vigorous scrubbing will remove the melted plastic from the bore.
Personally, I do not use plastic wads in my Black Powder shotgun loads. Instead I opt for separate card wads, the old fashioned way. Yes, it is a bit more time consuming to load a shotshell this way, but I don't have any melted plastic left in my bore when I am done with a match.
I use either a Winchester AA hull or a Remington STS hull for my Black Powder CAS shotshell loads. I use these specific hulls because the rules in CAS do not allow for ejectors, so I like the slippery plastic used in making these hulls so I can pop the empties out with a quick jerk backwards. I like the Remington STS hulls the best because they are the most slippery. However my recipe should work for almost any modern, plastic shotshell hull.
I opt for a slightly lighter powder charge than many others. My powder charge is 4.3CC of Schuetzen FFg. This comes to roughly 64.4 grains, or 2.35 drams. Different brands of powder will weigh a different amount because no BP weighs the same. I chose 4.3CC simply because that is the largest dipper in the standard Lee dipper set. I use 1 1/8 ounces of #8 shot, so this is considerably under a Square Load. But it does not blow the pattern and knocks down anything I aim it at. Of course, any size shot will do, as long as it is 1 1/8 ounces, but I keep #8 around for my Trap loads.
I use Winchester 209 primers, but any 209 primer will do.
The rest of my recipe is card wads that I buy from Circle Fly. Circle Fly wads are also available from Track of the Wolf.
http://www.circlefly.com/html/products.html
I put a 1/8" Over Powder Card on top of the powder, then I make sure it is slightly compressed by pressing a short length of 5/8 wooden dowel on top of the Over Powder Card. Next is a 1/2" Cushion Wad, then the shot, and finally before I crimp I place a .025 Over Shot Card on top of the shot. This is not really necessary, but because my powder charge is a bit light, my crimps tend to be slightly concave, and a piece or two of shot can escape without the Over Shot Card to keep them there.
I load all my BP shotgun shells on an old MEC Jr. I DO NOT use the powder bottle that comes with the MEC for powder. I manually dip the powder out of a coffee cup with my 4.3CC dipper. I will NEVER use a standard powder measure with Black Powder. I do use the standard 1 1/8 ounce shot bar with my MEC JR and have a large, oversized shot bottle installed. I also opted for the automatic primer dropper mechanism to speed things up a little bit.
Yes, it is a lot of work to make BP shotshells the way I do, but I have been doing it this way for a long time. Once I have everything set out and get cranking, I can average about 4 boxes of shells per hour.