I bought ten sheets of used 12mm rubber belt this week, cut to 900mmx1800mm ($270).
The plan is to hang them inside a steel box with a 40mm gap between them to allow bullets to fall into a tray at the bottom for recovery. The idea being that there will always be an undamaged sheet that will bounce the bullets off with no penetration. I'll probably make the back of the box out of 5mm mild steel just in case anything manages to make it through all ten sheets of rubber.
I stood four sheets up with 40mm gluts between them for some testing using CCI 40gn SV .22LR.
The CCI SV at the muzzle has 104ft/lbs energy and is entering the first sheet at 1080fps, and exiting at 735fps. At one-meter the bullets went through the first sheet, embedding themselves in the second, no bulge on the back but they're going to be hell to dig out of there. Two of the ten were sticking out of the surface and recoverable but the others were just below the surface. I'm guessing that means it takes 345fps of "energy" to pass through the 12mm sheet? Entering the second sheet, the bullet has enlarged from 0.223" to 0.34" so I expect it would burn up a lot more velocity trying to penetrate the second sheet.
At 25m they went through the first sheet and into the surface of the second sheet, mushroomed to 0.35". A couple were sticking out making for easy recovery, the rest are visible but not accessible. At 25m it has 91ft/lb of energy and 1020fps.
At 50m they went through the first and collected in a neat little pile at the bottom - perfect! At 50m it has 82ft/lbs and 960fps.
The bullets are very deformed, almost spherical and mushroomed to 0.34". At 50m the bullet is probably exiting at around 600fps or less, and bouncing off the next sheet. So I guess that tells me that the deformed 0.34" bullet needs at least 600fps to be able to penetrate the rubber at all? Surprisingly, they are not tumbling. I might try to find out what distance will stop the bullet from even entering the first sheet as that should tell me the minimum energy required for the bullet to actually penetrate the rubber. The CCI SV should be doing about 600fps by about 400m, so it'll be an excuse for some long-range .22LR shooting
Two sheets of 12mm rubber makes for a very portable backstop, so I'm thinking now of building one just for .22LR and one for everything else. For 50m .22LR it would only need to be maybe 600mm square. I haven't weighed them yet, but near as I can determine they should be around 14kg/m2 (these have no steel wire in them). Two 600mm squares are only 10kg of rubber. The full ten-sheet, 900x1800mm target is closer to 230kg of rubber, plus the structure to support it, so it'll have wheels and a draw bar on it.
I then stood up all ten sheets and fired two 173gn SP at it from the 7.62x54R. I haven't chroned these but I estimate they're in the 2400-2500fps range. From about one-meter range they plowed through seven sheets of rubber and bounced off the eighth sheet, both of them mushroomed to 0.61” diameter.
Don't suppose anybody else has tried something similar?