YOu need to read the V.M. Starr article on Bob Spenser's Black Powder Notebook to learn about using Black powder shotguns.
[url] http://members.aye.net/~bspen/starr.html[/url]
Unless you are using plastic shotcups to protect the shot, you are not going to get the tight patterns, even with a full choke " Jug choke " in your gun, at those velocities. You may get those velocities using Goex 3Fg, and Swiss 3F powders, but all that speed is lost in the first 20 yds. Check the tables in the Lyman Shotshell Reloading Handbook, which I have studied at great length trying to improve down range velocities with both modern shotgun shells, and with my black powder shotguns.
IMHO, you are much better off doing as V.M.Starr recommends and keeping your loads below the speed of sound( 1100 fps) at the muzzle. At the 20 -35 yd ranges that your gun will pattern best with shot, you get good on-target energy from the pattern, and can kill game. I don't believe you are going to make a 60 yd. killing machine out of any ML shotgun no matter how much powder you stuff in the barrel. All you accomplish is blowing patterns.
Now, if you are using plastic shotcups( wads), and that gun is full choked, you can get patterns that approach the full choke patterns you get shooting modern guns. And, with choke, you can extend the killing range of your patterns. Without protecting that shot from rubbing flats on pellets as the column travels down the barrel, you just won't get the patterns you expect out of a modern gun and loads. Too many pellets develop flats, and fall out of the pattern within the first 20 yds. The smaller the gauge you use, the more pellets rub against the inside of the barrel, and the fewer pellets remain in the core pattern.
Your quest for 1300 fps seems to be irrelevant. I suspect you would not even be bothering except fot the fact that you spent the money to have a full jug choke cut into your gun. I wish you well, and hope you will give us a report on what kind of patterns you are actually getting with the gun.
I have a full choke barrel on a modern gun, and after spending enough time on patterning shot out of it, have settle on a "light " 2 3/4 dram, 1 1/4 oz. #5 shot for my hunting load. It seems to deliver the most shot in the patterns at the longer ranges. The velocity is well below 1200 fps, and probably below 1100 fps.My 20 ga. Fowler load is in the 1030 fps. range, but the gun has no
no choke. It actually throws a very good pattern at 25 yds.