Gammon,
Where my rounds land, they are impacting the rubber directly. Like I said, I'm sure there is metal back there, but actually the rubber does an excellent job. I get what you are saying though. If you shoot higher up, where the rubber is thinner, the bullet may impact the metal directly.
But you are missing the point if you don't realize that rubber itself makes an excellent backstop. I wasn't a member prior to the rubber being used at Loeb but I suspect that the metal stop is still adequate, but by putting the majority of the rounds into rubber, they greatly reduce the toxic waste created by the range. This is because the in-tact bullets that rubber allows can be recovered and easily recycled. This is in contrast to the dust and massively fragmented bullets created by a steel stop.
The beauty of the shredded rubber backstop is that there is no backsplater. I've taken classes at ranges with rubber backstops where we were shooting ARs and handguns at contact distance to the targets with only 5 ft to the backstop.
These backstops have become the standard for "tactical" type backstops that can catch anything from any angle, safely, with no danger of ricochet, and no dust.
I've used this material in a 6 gal taping bucket to catch handgun bullets when I function check pistols or ammo. I bought a few bags of the shredded rubber landscaping material from Home Depot and simply filled the bucket with it. It works great. The recovered bullets look like they could be reloaded and used again.
Here is a company that sells the rubber for just this purpose:
Ballistic Backing Rubber Fill from RubberScapes
Here is a company that makes self contained bullet stops using shredded rubber:
The Bullet Bunker - The bullet stops here
Here is a description from a firing range.
Bullet Trap: Unlike the old, out-of-date ranges located in the greater Knoxville area, Frontier has a modern, safe bullet trap system made of three feet of shredded rubber that gently catches your bullets whole - eliminating bullet fragmentation, lead splatter, and dangerous bullet bounce back. In other words, when you fire into our backstop your bullets do NOT break into hundreds of little fragments or create clouds of deadly lead dust after smashing themselves to smithereens against steel plates. Instead, at Frontier each round fired is gently "caught" or "captured" in the rubber backstop, with little or no deformation! This means there is harmful lead dust is not sucked into the ventilation system and then spewed-out into your breathing space at firing line. Indeed, ranges that allow your bullets to smash against steel plates are knowingly creating deadly hazardous waste conditions that must be controlled and regulated by the EPA and OSHA. But not at Frontier! Indeed because we adopted the latest air quality standards and range design criteria, expended rounds in our range become a simple recycling situation. 
