Check here for one place:http://www.chapelsteel.com/ar500-ar500f.html
Most AR (Abrasion Resistant) plate is not made to an ASTM or AISI spec. If there is one, I am not aware of it. They are usually "in-house" specs. I have never dealt with AR500 plate, but my company does use AR 360/400 plate. It is VERY difficult to weld, cut, form, etc. It is extremely brittle. In fact, that is how we cut it. It is put in a press brake and it is "bumped". The plate breaks along the line instead of bending. It is a clean, straight break. The specs I have seen do NOT guarantee any yield strength, ultimate strength, minimum elongation, etc. simply because it is not used for that kind of structure. I kind of doubt that it is best for firearms target baffles, but I have no actual experience in using it for that.