It is a woven fabric which the outside layer of the bag seems similar to geo-synthetic fibers used as either road stabilization or heavy duty weed barrier. Don't know what is inside as the unit is like an inch or more thick. Assume it is more layers. I am shooting woods and carbons with no trouble. It absorbs the energy of an impact, and doubt it would be an issue on aluminums. I use it in front of a chain link fence outside or placed 1-2 feet in front of a concrete wall inside.
I'm hoping Dave (Vermonster13) will chime in. I know he also uses one, and if I recall correctly, was as pleased as I am.
Keep in mind, the idea is to hit your target....
for me it was the peace-of-mind if one gets away. I have had the fence eat a few arrows in the past. Also, arrows into the neighbor's yard is unacceptable......