Only recently in the last couple seasons have I started hunting birds with a flintlock smoothbore ......but I've done ALOT of testing with various loads trying to find one that is truly satisfactory to 30 yards out of a 20ga cylinder bore.
I've been a turkey hunter for a long time and have quite a bit of experience and success with Hevi-Shot in modern shotshells and what they will do out of quality chokes is nothing short of astounding when compared to lead.
I've tried so many different lead loads in my flinter with conventional thinking in materials and volumes and none of them I would consider 100% beyond around 18-20 yards max on these big tough birds.
I made a personal decision to try some of the Hevi-13 pellets to see if they would work some of the magic they do in modern shotshells. I have dozens of these laying around so I started cutting them open and robbing the pellets for testing.
In order to try them I also had to allow plastic shotcups into my shooting in order to protect the bore from these extremely hard pellets.
The shotshells I had on hand to steal from with the smallest dia. pellets possible were some "Magnum Blend" Hevi-13. These contain a mix of #5,6 and 7 pellets. I hand sorted these pellets to break out the 6 and 7 sizes only.(I've handled the Hevi-13 pellets alot and this brand is round for the most part with a good number of "teardrop" shapes and some snowmen as well)
I use the Ballistic Products 20ga plastic shotcups that will hold 1 1/4oz volume of lead shot.
I cut these into 4 even petals and gave each a slight and equal bend backward prior to loading to ensure that they peel off the shot column relatively quickly upon leaving the bore and offer no chance of flipping whole and turning into a slug in flight.
This picture is of one of the test shots with an even mixture of #6 and #7 shot at 25 yards....I shot multiple patterns with this load and the results were very consistent even when pushed to higher velocities with the hot 3F loads.
As long as I shoot straight I would consider this 100% performance for shooting gobblers in the head/neck at 25 yards. In reality probably 30 yards.
I've always used a minimum of 100 pellets in a 10" circle a standard for reliable maximum distance for turkeys.
This load is what I hunted with last season but didn't get a bird in range to drop the hammer so I didn't get to "field" test it
I am going to test much more this spring prior to the season and will probably center that testing around the #7 pellets only. These pellets although small hit harder and penetrate better than lead of a size or 2 larger.
This will allow me to fill in even more of the pattern with higher numbers without sacrificing performance.
I have a feeling that 1 1/4oz (equivalent lead volume) of straight #7 Hevi-13 and 85-90grs of 3F will be a real turkey knocker and probably give me that honest 30 yard pattern that I can count on.
One other note.....Hevi-13 isn't tremendously heavier than the same volume of lead shot. 1 1/4oz of lead #6 from my dipper weighs 545grs give or take a couple pellets.
The same volume of Hevi-13 #6 shot weighs 592grs give or take a couple pellets.
So in reality you're not shoving an unreasonable increase in mass weight vs lead.
The 20ga plastic shot cup adds another 28grs to the finished load.
Together they are comparable to the mass weight of a little less than 1 1/2oz of lead by itself.
This gives me the desired combination of high velocity for penetration and great pattern density to put those pellets on target.
It's not for everybody or going to appeal to everybody's thinking or sensibilities but it's another option if you want to maximize a cylinder bore gun.