i'm looking at have some custom adapters made but not sure what grade steel is best?
If someone is going to machine them special for you , just mild steel would work fine .
I make my own using 308 stainless steel but I'm using scrap stock and it doesn't cost me anything .
12L14 carbon steel will be the easiest to machine, and work well. Any low carbon steel will work.
What about 4140?
That would be fine, good though steel.