Seems like that hard lead will put a bunch of unnecessary wear and tear on a Forsyth rifled barrel. After all one of the reasons these barrels aren’t more popular is because of the narrow lands, they wear out faster.
Hungry Horse
I think perhaps these barrels aren't popular (yet) is because most guys don't hunt large game or at ranges that requires heavier loads.
The slow rate of twist normal for true Forsyth-type rifling, usually shoot best with more powder than shooters today are willing to use.
The so-called Forsyth rifling of today is too deep in my opinion to truly benefit from the narrow lands and wide grooves & slow twist.
With a slow twist, the rifling need not be deeper than about .006" which will allow a substantial patch to take the 'heat' so to speak, coupled
with an easy loading ball 1 bore size smaller, maybe 2 bore sizes smaller in the 20 bore.
As for them being not as long lived as any other rate of twist, I don't believe that for a second. Who here has worn out a barrel? I mean really
worn out the barrel, not just shot one slick, as our late friend Roger Fisher did, with 45,000 shots in a Don Getz bl. Roger roughed the barrel a bit
and it was back to winning again.
I would think a true .620" with Forsyth rifling, might shoot well with a .595" (22bore) ball & a .024" canvas or 10 ounce denim at .021". That is, if the
rifling is .008" or shallower. It might even work with a barrier between powder and patched ball, with an 8 ounce denim patch.