One of the reasons folks urge using a 20 gauge black powder gun or larger, is because of the lack of a choke in so many of the guns, the amount of shot needed to take the turkey is high. The pellets have to be large enough to penetrate, which reduces the number of pellets per ounce, so the bore needs to be large enough to have a decent amount of them.
I like #5 shot for squirrels in my 20 bore trade gun, and I like an 3/4 to one ounce of shot. That's about 125 to 170 pellets. 3/4 of an ounce is about the same weight of a single ball that I shoot, so I know I'm OK with 3/4 of an ounce to an ounce, on squirrels. YET most of the turkey hunters out there will tell me that's too light for turkey. I may need as much as 1.5 ounces of shot. :shocked2:
In your case, a .570 ball is about 279 grains of lead, or 5/8 an ounce. That's about 110 pellets of #5. You could, I suppose increase the shot load by the same percentage that I increase mine when going from 3/4 to 1 ounce, which would give your .58 3/4 of an ounce of shot for a "heavy" load...and 125 pellets....for squirrel. What about turkey? That probably isn't enough to match a 20 gauge at the max range of the 20 gauge. So... do you have to "match" the 20 gauge performance?
I think that the answer is going to be, if a legal gauge in your state, that you will be able to hunt turkey, but you're going to have to get them in under 20 yards to use the .58. That's how you're probably going to compensate for the lower amount of pellets. That's supposing that an unchoked 20 gauge at 25 yards is being used as a "standard" for hunting turkey with a smoothbore flintlock. So just be aware you are probably giving yourself a bit more of a challenge.....
...., but if we didn't like challenges, we wouldn't shoot flintlocks, now would we? :haha:
That's also assuming you don't match the amount of shot that the 20 gauge is shooting....some folks would do that, and I can't say that's a bad idea, as I don't know how your barrel is made. It might be perfectly alright. In such a case then you should be fine at 25 yards.
LD