Mr Browning considered the A-5 his greatest achievement - it would fire black powder or nitro. The clean up might not be fun if shooting BP.
As Hawg refereed to, the buckshot has to be the right diameter so it will lay flat, and some buffer included to keep the balls in place [ or cards above and below the BS]. Other wise the BS will try to creep into the empty spaces and come out deformed giving a poor, or no pattern.
Maybe the best solution, if you just want to shoot a BP shotgun, is to reload modern shells with BP. All you need if you don't have a SS press is a big nut for the shell to sit on , and a dowel with a finish nail sticking out the end about 1/4 to 3/8". Stand the shell up on the nut, insert the dowel and hit it - it's now deprimed. Put a primer on a flat surface, put the shell over it with another dowel inside and hit it. You're now primed. From Precision Reloading or BPI you can buy a cheap adjustable measuring tool for shot and powder. With BP you normal use equal amounts of powder and shot. Fiber wads can also be bought from the above or you could make them. Powder first, a hard .125 over powder card, a 1/2" soft cushion wad [ or what ever thickness you need for a good stack height - trail and error ], shot, and a .028 [ you want thin ] thick overshot card. If you have a press you could fold crimp it and forget the OS card. Or buy a roll crimper, or just glue the overshot card in. Those cushion wads can be sliced with a thumb nail for different thicknesses for proper stack height for a good crimp.
Most 12ga loads are anywhere from 70 to 80 or 90grs of 2F, 1oz to 1 1/4. The load I use the most is 1oz, 75grs of 3F. But then I'm getting a little long in the tooth and don't like a lot of recoil. Also most my shooting is claybirds.
I run the shotgun venue at my club and put on a SxS event every year. Some of the guys shoot BP and we have an award for it because it's so much harder - on doubles many times you can't see the second shot. At this time I have a Lefever 10ga, three Parkers - two 12s and a 10, nine Remingtons - one is a 10 the rest 12ga guns. Oh ya, a little Dickinson 410. All the guns are Damascus except three Remingtons and the 410. I limit my BP shells to SxS events.
Remington never got the name like Parker or LC Smith, but they and Ithaca built some of the best guns. Open one up and see how many fewer parts are being used. The only thing I've ever had to do to my Remingtons is replace the top lever spring on two of them.
Find yourself a cheap used single or double barrel gun and give it a try.