When its time to replace the backer board. Its a good time to collect some lead.
Shovel it right out the hole onto the ground. Then sift it back into the trap.
Remelt and repeat.
Yours is quite a bit bigger than mine, and probably works better for the rifles. For my revolvers and pistols I just use some recycled 6 gal buckets I get from work. They stop most anything I shoot at 25yds but there are a couple that I have to move out to 50 to keep from blowing out the bottoms.
These are easy enough for us to roll onto the tailgate and sift out back at the house. It only takes a half hour or so to sift and then pour back into the same or a new bucket depending on how well I was shooting....
Brilliant minds think alike.
My home made miniature version for airguns looks very similar, but without the need for the sand.
Thanks for the pictures, they're going into my range equipment files.