The absolute best thing I have ever seen is conveyor belt used in rock quarry conveyors to move rock....If you can get it. We use it to face portable bullet traps made of Brinell 550 steel. It's very tough and will last for thousands and thousands of 5.56 rounds and pistol rounds. It has an almost self sealing quality after the round passes through it.

I don't know if you can source it though. We get it from a local big name quarry company. We had to do a lot of reassuring that we would dispose of it properly when it was no longer serviceable. The surface of the belt as well as the cord inside is number coded and traceable to the end user. It's used to heavily fine the company if it's found illegally dumped.