It's been many years ago so my memory might be off a bit, but I did shoot a .62 caliber Jaeger style rifle for a while. The barrel on mine was 36" long with a 1-72" twist rate ad a true .62 caliber grove depth.
This rifle shot best with balls of .610" diameter and denum patches (.225" if I recall correctly). Smaller balls or thinner patches killed accuracy. You "might" get by with .600" balls but your patches will have to be VERY thick to seal the groves properly.
I loaded this rifle with FF powder charges up to 160 grains, but finally settled on a "hunting" load of 110 grains of FFF powder. I chose FFF mostly because I could use the same powder to charge the pan on my flintlock. Load charges with FFF will be a bit lighter than with FF due to greater pressure being produced, but the velocity will be the same.
Any load less than 80 grains seemed to reduce accuracy quite a bit in my rifle and loads above 120 grains became uncomfortable with little gain in velocity. If I recall correctly, my 110 grain load of FFF gave about 1600 fps.
Trajectory with this load was very nice out to 100 yards (as far as I ever shot at game). With zero at 25 yards the big balls hit about 4" low at 100 (or as we used to say..."minute of coffee can")...well within the kill zone of a whitetail.
Those big balls really don't need a lot of velocity to kill well as they rely more on momentum than energy to penetrate. With round balls the drop in velocity at any distance over 50 yards is such that any attempt to gain a flatter trajectory with more velocity is a losing proposition.
I'd suggest you try to get some .610-.615" balls with a heavy patch and a load of 90-120 grains of powder. If you use FF grade powder....maybe 100-140 grains. Somewhere in that range should be the "sweet spot" for your rifle.
If you "have" to use the smaller .595-.600" balls you will need a VERY thick patch. Might consider a thin (.35-.45") patch of leather (as was used sometimes in original rifles) to make the undersize balls seal properly and avoid burn-through.
You are going to love the .62 caliber.....it's the "magnum" of the round ball age as the only effective way to gain "power" with round balls is to step up size.
