As I was looking forward to this opportunity,and others here,suggested it too,not long ago,at the last reenactement,i was at,members from a Hungarian reenactment group brought one of their battered shield and ask me to sling at
So i slung about 10 stones from about 15m at it, till i got it with a 210g quartz stone. Before this i was slinging lightweight safety ammo at the dacians so,i had to readjust to heavy stones.
The shield was 8-10mm plywood(have to ask) and held up only with a thin stick.The sling was 90cm long.
With a long sling,pro slinger,and oval shaped stone a very similar damage could be produced at long ranges(~150m)
The shield is more or less like the originals in it's caracteristics IMO.