For rimfire you don't need AR500 or 400. So many people really don't understand what this steel is really for. AR stands for Abrasion Resistant, and the number is a "hardness" number. I have a grapple on the front of my tractor and there is a huge sticker on the side of it, AR400
In a nutshell, "earth engaging" tools will generally carry an AR rating....the better ones anyway. You ever look at a blade on a bulldozer, you will see a little strip of steel on the bottom of the blade, that is hardened, and that is the part doing the "digging" the rest of the blade is of a "softer" steel.
"Good" steel and "hardened" steel are not one in the same, you can have very good steel that in the job it needs to do is not hard.
Ever sit at a RR crossing....sure you have....see the rails move up and down as the trucks roll over it, sure you have. The rails and switch plates are soft steel....the rail has a layer of hardened steel where the trucks ride, but the rest of that rail is fairly soft so it will bend under the weight of the train. The switch plates are the same.
Story time, a two for one:
I have a bunch of RR spikes, a guy behind me is dabbling in playing black smith. He sees the spikes and asks can I have some of those, sure, but what are you going to use it for. So and so use to work for CTX and I think it would make a cool gift if I made a knife out of one. That would be cool, but it will be a crappy knife. Why is that, the steel in the spikes are very soft. The RR uses cheap steel? I never said it was cheap, I said it was soft, the steel the spikes are made out of is just going into wood with a little "lip" on them to hold the rail, it does not need to be hard....hard and good are not equal.
He did make a cool knife, but it would not hold an edge.
In the rimfire world you can use very mild steel and not have issues.
Personally I only buy AR500 steel targets now....why, because when you are old and don't remember if that cool spinner you have that has a coyote on one end and a wood chuck on the other is mild or hard, So you send a 3006 and the target does not move....hmm, nope you hit it you just blew a hole right through it.
Years ago when the surplus 8mm stuff was really coming in I had a couple people out and we are banging steel till the rifles get too hot to touch, I don't know how many round we shot but a lot, and the steel showed no real damage. In that case of loose 8mm rounds is one round with a black tip.....is that AP? Yea I think so. Can I shoot it at your targets, yea I want to see what it will do.
Went through it like it was paper.....really crazy stuff.