I would not even try using Broadheads for target practice, even with field points it's very hard to stop these arrows,
I ended up using blunts for practice and I had to put together a target with some foam then a bag packed tight with old denim and finally a 3/4" ply to stop the bolt, ence the blunt so it wont stick in the wood,......I can link a bunch of threads where I was testing backstops,
but I can tell you the first time I shot a bolt at half power out of my Dragon Claw .50 into bales where I use to comfortably shoot my 90 # longbow and 1000gr arrows,....it just blew through them like butter, I was shocked.