If I am understanding you post correctly, you are wanting a BP cartridge handgun? If so . . .
You refer to an "open top" that you saw but it was chambered in 38? If so, it was probably the Uberti 1872 "open top" chambered in 38 Colt Long/38 Special - which is designed for smokeless. However, it will shoot BP cartridges just fine.
The nice thing about those revolvers is that the barrels are bored/rifled in the standard .357 bore as are modern 38/357 revolvers. i.e. you can shoot a standard lead slug out of them and do not need a "heeled" slug. If it is chambered in 38 Colt Long/38 Special - you actually have three cartridge choices. 38 Colt Short, 38 Colt Long and 38 Special all of which can be loaded with smokeless or BP.
I load and shoot a lot of 38 Colt Short/Long and 38 Special. If you load your cartridges with BP, you must use a "compressed load". I use a 38 Colt Long casing to measure the charge of 3F BP for the 38 Spl. cartridge. For the 38 Colt Short and Long, I load the casing to a level about 1/8" under the top of the mouth of the cartridge casing. When seating the lead bullet, it does the "compressing" of the powder just fine and I have never bulged a casing doing it this way. For 38 Colt Short/Long I use either a 120 or 150 gr. lead bullet - on the 38 spl. I use a 150 or a 158 gr. lead bullet. Of course the bullet must be lubed with a BP lube to keep the fouling soft.
Loading BP 38 cartridges is really not hard at all and it's a lot of fun. You can experiment with different bullet designs and weights. Normally, I cast my 38 bullets out of "range lead" for smokeless loads but for my BP cartridges, I use "soft lead".
Earlier this past summer, I was going to order one of the Uberti "open tops" in 38 but I got "waylayed" and ordered a Uberti 357 Bisley instead. It shoots BP 38 Colt Short/Longs, 38 Spl. and 357s just as well as the "smokeless" cartridges in same configurations.
I have shot BP cap and ball for 50 + years but I enjoy the BP 38s even more!
