Mike,
We'd put up a board backstop to staple targets to, and behind this, another row of posts and fill the gap with first cut boards. After a shoot, we'd take these boards out and pick up the loose lead, and pry out what we saw.
This was OK, but after a few shoots, the planks got like matchwood.
For the muskets at 50 metre/yd, I use a piece of a round baler, flat steel maybe a bit more than an eighth thick, propped up at an angle to the targets and behind the target frames. This deflects the balls down over into a thick log. Idea being when the log gets 'tender' we can salvage the lead.
Not at all perfect or best, but what we have at present.
Jason has made a bullet catcher out of pipe with an offset slot in it, with wings on the outside to funnel balls in. Off-set slot means they spin around in there and don't come back out. Seems it'll even catch HV stuff.
Best ,
Richard.