I remember using some stuff in a silent pellet trap which was much firmer than putty, plumber's mait, plasticine, modelling clay, or anything else that's been suggested. I find these softer meterials unsattisfactory as you have to keep remoulding it every time it becomes to riddled with holes, and after a few hundred shots replace it as it has more lead than anything else and doesn't mould well.
I have no idea what the stuff I used was, nor where it came from. It was some 20 years ago. It was like very dark silvery grey modelling clay - imagine plasticine after being in the freezer? It wasn't sticky and was very tough work moulding it into the back of the baking tin. I used about a 3cm depth, but much less would have worked.
The best thing about it was that whether I used my CO2 BB gun, or a 12ft-lb rifle, or whether I used pointed, rounded, or flat pellets, the penetration was only a couple of milimetres at most. They just splat and stuck on the surface, pretty quietly. Eventually, after a few hundred pellets, they would cake up and you could just bursh them off with hardly any effort.
It could well have been proper ballistic putty as, if I remember correctly, the guy who gave me the pellet trap was a ballistics guy at some physics lab who did forensics for the police. Unfortunately I lost contact with him over
15 years ago.
Wonder what it was...
Adam