I was thinking about making one, but couldnt find much info on home made ones..I dont have any AR steel, just 3/8 mild. Could I make some out of this and what caliber/yds would it be good for.
I really dont know much about them. Any help????  

i use old rail road tie plates welded to a chain

Take a look at speedwell targets and see how they make theirs.
As for mild steel forget it it will end up cupped and look like the surface of the moon. Making it ever more dangerous.  500brinnel hardness is the standard for target steel you want to mount your target in such a way that you can control the direction of your projectile ie. into the ground or a earthen burm ect. Think of it as a bank shot  or controlle ricochet.

3/8" mild steel will hold up to pistol rounds pretty well.
I've got a couple that have a few hundred rounds on them, and they are still in good shape.
AR500 is the stuff you really want, but it is pricey.  For the price you could replace
your 3/8" targets ten times and still be out less money.  
As with any steel targets, you want to shoot from a safe distance.

Thanks for the replys .....fellas